There are lies, damned lies, and statistics. There are also historic counties, postal areas, and administrative counties.
OK that opening paragrapht doesn't really work and the second sentence is uglier than I would like but it sort of illustrates the point. Anyway lets get back to the subject which is whether Lower Edmonton is in Middlesex or London.
It doesn't exist, you all cry. Not so! Just because at the end of the 19th Century someone invented the London County Council and the Middlesex County Council and then in 1965 the Greater London Council came along and they were trashed, it doesn't mean Middlesex doesn't still exist. It also doesn't mean that Middlesex was the area covered by the said council anyway.
Similarly if you consider Middlesex does still exist just because the Post Office managed to keep Middlesex post codes for many places that doesn't mean that those areas are all that is left of Middlesex. The postal areas are an administrative convenience for the Post Office and the London postal areas were defined long before even the above councils were invented. The postal areas actually include a few areas that aren't Middlesex at all (for example Elstree and chunks of Barnet).
The name Middlesex comes from 'Middle Saxons' and ties in with the kingdoms of Essex, Wessex and Sussex. It isn't clear if it was ever a kingdom in its own right and one of the more convincing suggestions I have seen is that it was originally part of the Kingdom Of Essex until Alfred The Great liberated it from Danish invaders in 886, at which point it became part of the Kingdom Of Wessex until the Norman conquest.
The eastern boundary is clearly established as the River Lee (or Lea if you prefer). It is a subject of historical speculation as to whether Middlesex also embraced Surrey, much of Hertfordshire and some of the other Shires. The name Surrey could suggest it was the southern part of Middlesex and the word Shire could mean those areas were 'shorn' from Middlesex. As is often the case, the Domesday Book of 1086 is useful for drawing lines under things and it was from there that the word 'County' derives (a county being controlled by a compte or count and being a unit of account!). So Middlesex ends up with the River Thames as the southern boundary and northern and western borders roughly where the Shires are now.
Something you will note from the above is that basically all of the north of London west of the River Lee (which joins the Thames just east of the Isle of Dogs) is part of Middlesex. The City Of London is basically its county town. There is a saying that to be a true Londoner you must have born in the square mile of the City Of London. Not in the square mile and you are really from Middlesex, Surrey, Essex or Kent. Some would go further and also require Bow bells to have been ringing when you were born.
Middlesex was once divided into six 'hundreds' of which one was called the Edmonton Hundred (hence the Edmonton Hundred Historical Society). This covered the parishes of Edmonton, Enfield, Monken Hadley, South Mimms and Tottenham. It seems that somewhere along the line it was the parishes that became the focus of things and the Parish of Edmonton included not only all of modern day Edmonton, but Southgate, Winchmore Hill and Palmers Green too. So that places Lower Edmonton very much as a Middlesex village.
In 1857-8 the Post Office implemented their 'London' postal areas with Edmonton being part of the "N" area (the sub-division into numbered districts such as N9 came along in 1917). They were however an administrative device for their own purposes and did not represent any broader definition of London. Edmonton was part of a London postal area but still very much part of Middlesex.
The Local Boards Of Health became something of the focus of local government by the 19th century and in 1881 Southgate (including Palmers Green and Winchmore Hill) separated from Edmonton and gained its own Local Board.
In the 1890s there were various reforms of local government and it was at this time that 'administrative counties' were formed. It was always acknowledged that they were for the purposes of local government and administration and did not replace the historic counties (aka 'ancient', 'geographical' or 'traditional' counties) whose borders they did not always follow, though it seems they did at least mostly keep their names and everyone basically understood what was what. It is a pity they called them counties instead of using another expression that would have been so much easier in the long term. Over the years they have basically taken precedence and the OS maps feature the administrative boundaries and not the true historical ones. This has become a bigger issue as a result of several reviews of local government since the 1960s where 'counties' have disappeared, reappeared, been invented, and generally messed around with to the extent that everyone is thoroughly confused. The Association Of British Counties will provide more background.
It was at this time (1890s) that the London County Council was created out of large chunks of Middlesex, Essex, Surrey and Kent and it was intended to reflect how the towns and villages surrounding the Cities of Westminster and London had rather fused together into a larger conurbation and were obvious candidates for common administrative control. At the same time the Middlesex County Council was created from the bits that were left over. Edmonton got an Urban District Council within Middlesex County Council. So Edmonton was still very much Middlesex, and of course the old Middlesex had never gone away, it was just that bits of it were administered under another name. The LCC covered only as far as Islington and Hackney to the north. Tottenham came under the Middlesex administration too.
Edmonton became a municipal Borough in 1937 but was still under the Middlesex County Council. However in 1965 a further review of local government reflected the existence of a de facto 'Greater London' and the Greater London Council (since abolished) replaced the LCC and the Middlesex County Council. Edmonton became part of the London Borough Of Enfield along with Southgate and Enfield (original proposals bracketed Edmonton with Tottenham which would seem more like with like community wise) and Middlesex only remained on the post codes which the Post Office mostly left alone. The historical county remains though...
I'll try not to repeat the above discussion. I'll just give my definitions of London, and yes I mean definitions plural - there have to be several.
'London' can mean the City of London, the 'square mile' itself. However it tends to be referred to as 'the City'.
When I say I am going 'to London' or worked 'in London' I tend to mean the 'West End' which is actually the City Of Westminster. This seems to be consistent with the way most people talk. The expression 'Westminster' tends to be used just for the area around Parliament rather than its whole.
'Central London' is pretty much the same as the above but can include the City too. The centre of 'London' is considered to be Charing Cross which of course is actually in Westminster.
'Greater London' is pretty well understood as the metropolitan area but I can't say I ever see or hear it used to any extent. It too gets called just 'London'. The only real usage was in the name of the former Greater London Council and the more recent Greater London Authority (not a body I had any real awareness of but it incorporates the Mayor Of London and the London Assembly which I did know about).
I once tended to say Edmonton is in 'north London', though these days I am more likely to say 'north-east London' as that is how transport maps tend to consider it and otherwise people think you mean Hampstead or somewhere like that.
Well there can be no doubt that Lower Edmonton is in Middlesex. The borders of Middlesex are essentially as they have been for a thousand years or more and they always will be. So do I say it is? Well unless I am doing something like a family tree where I find the historical counties essential for providing a fixed geography over time, no I don't.
I was brought up to know my address as ending 'Lower Edmonton, London N9'. Somewhere along the line the Post Office lost the 'Lower' bit, and in fact the whole 'Edmonton' bit usually vanishes too which is a shame because post always seems more reliable with the town specified . However I digress. The point is that has always led me to think of it as London, and north London at that.
I have also been aware of the other areas with Middlesex addresses, in particular because the parts of the London Borough Of Enfield that weren't in the old Parish Of Edmonton still have them. The influence of this was for me to long assume that the 'Borough Of Edmonton' I had seen on old street signs (I was an infant when the London Borough of Enfield was created) had been part of the London County Council and not Middlesex and it also led me to think of Middlesex as very much to the north and west of London and although I knew it was bigger in the past and Edmonton was in it I had never realised its full extent.
I also have to be practical about things. If I said I had lived in Middlesex people would think of the north-west or westerly parts of London (despite the nearest place with a Middlesex address being less than a mile away from Edmonton) or not know what I meant at all. So I am a Londoner. I am a Middle-Saxon too but I don't really feel like one.
I sometimes find it convenient to consider Lower Edmonton as having been a village in Middlesex that became a town in London.