Metropolitan areas are, in the most literal sense, the big ones – not simply a city itself, but its suburbs, commuter towns and rural hinterland. On this definition, London isn't Greater London – it's a large chunk of the Home Counties, too.
The figures below are from a document published in 2007, and are based on data taken from 2001, so the numbers are pretty out of data (hence the inconsistencies with the other lists above). But it's the best we've got so here, courtesy of the EU's ESPON project, are the top 10.
1. London – 13,709,000
2. Birmingham-Wolverhampton – 3,683,000
3. Manchester – 2,556,000
4. Leeds-Bradford – 2,302,000
5. Liverpool-Birkenhead – 2,241,000
6. Newcastle-Sunderland – 1,599,000
7. Sheffield – 1,569,000
8. Southampton-Portsmouth – 1,547,000
9. Nottingham-Derby – 1,543,000
10. Glasgow – 1,395,000
Considered as a metro, rather than a city, Birmingham is way ahead of Manchester – a result of its better transport links to surrounding towns, perhaps. The twin cities of South Hampshire are back in the rankings, and several other cities look a lot bigger when the whole of their economic footprint is taken into account.
Glasgow, however, doesn't: it barely makes the top 10. Compared to cities like Birmingham or Leeds, it doesn't have much of a hinterland.
But that isn’t the only way of visualising a city: this is part of a longer article exploring different definitions, and including different rankings.
You can read the whole thing here.