London is often regarded as everything inside the M25 motorway. Not bad as a rule of thumb, but not that accurate either.
Here's the map of Greater London (shown in the lighter beige/brown colours), with the M25 orbital in blue and everything else in the murky orange.
As you can see, for most of its borders, Greater London stops well short of the M25. This means that there is a considerable population that finds itself inside the motorway but not part of the big city. It's a psychological buffer zone between London and the Home Counties, simultaneously allied to both. Mind the gap.
How many people live inside the M25 but outside London? We can make a good guess by looking at the home country districts that encircle Greater London. Reliable population counts for these are readily available — we just have to estimate what proportion of each is inside the motorway.
Here are the 21 districts that have at least part of their territory within the M25, along with estimated populations inside the motorway.
Watford: The district lies entirely within the M25 and has a population of 90,000.
St Albans: While the town of the same name is firmly outside the motorway, the district extends a little into our territory, taking in the village of Bricket Wood and a few hamlets. Estimate 5,000.
Hertsmere: about three-quarters lies within the M25, including most of the main population centres (Borehamwood, Bushey, Radlett). Of the entire population of 102,000, let's estimate 70,000.
Epping Forest: A sprawling district, perhaps one-third inside the M25. Estimate 60,000.
Brentwood: Only tiny pockets of rural land intrude within the M25, so the population here is effectively 0.
Thurrock: A small but well-populated area of this Unitary Authority around Purfleet is within the M25. Estimate 10,000.
Dartford: About two-fifths of this district lie within the M25, but these are also the most densely populated. Estimate 60,000.
Sevenoaks: This large district is only partially within the orbital, with Swanley the only town of any size. Estimate 20,000.
Tandridge: About two-fifths of this district lie within the M25, including the significant towns of Warlingham and Caterham. Estimate 31,000.
Reigate and Banstead: This district is split roughly 50/50 by the motorway, but the biggest population centres are outside. Estimate 30,000.
Epsom and Ewell: Entirely within the M25. Population 78,000.
Mole Valley: Only Ashtead creeps over the motorway. Estimate 14,000.
Elmbridge: Almost the entire borough lies within the M25, including Esher, Weybridge and Walton-on-Thames. We can be quite confident of an estimate of 133,000.
Guildford: There's little more than a farm inside the M25, so we'll call this 0.
Woking: Only the village of Byfleet reaches inside the motorway. Estimate 8,000.
Runnymede: A fair chunk of the district counts towards our total, including the towns of Addlestone and Chertsey. Estimate 35,000.
Spelthorne: Another district entirely within the M25, save for the Wraysbury Reservoir. As ducks don't count towards our tally, we can be reasonably sure the population is about 98,000.
Windsor and Maidenhead: This unitary authority dips the tiniest of toes over the M25, and contributes nothing to our total. Estimate 0.
South Bucks: A fair chunk of land, but largely rural in character save for Denham and its offshoots. Estimate 8,000.
Chiltern: Another of those districts that seems to creep inside the M25 just for the fun of it. There's nothing here except a field. Estimate 1 scarecrow.
Three Rivers: By contrast, this district is half-in, half-out, and perhaps the trickiest to assess. With Rickmansworth, Moor Park and South Oxhey, there's definitely more 'in' than 'out' action here. Estimate 70,000.
If we now add the estimates together we get a grand total of 820,000 people. Most of these stats were based on 2014 figures. With increasing numbers of people moving further out of the centre to afford property, it's likely that this figure has now reached 850,000.
So, there you have it. The equivalent of the population of Newcastle lives outside London but inside the M25. The next question: what do we call the inhabitants of this notional, interstitial city?
Photos by the author.