Which cities in the UK need or don't need a fully-fledged Underground/metro system?

[Fazaar1889]

Birmingham and Manchester seem like they may be the best places for a metro; they're massive with growing populations, however, it could be argued that Manchester's tram system is so extensive that a metro isn't necessary. Leeds is easily identifiable as a massive gap, but what about Nottingham and Sheffield? They're not the biggest cities but they're not exactly small either. Bristol seems to be in talks... Does Glasgow need to extend its subway system?

Also, outside of the biggest cities, are there smaller ones like oxford that need a tram?
What about Large towns like Reading or Guildford? Are there major bus routes that could be replaced?

 

[swt_passenger]

I think this has just been done in speculation:

Where else in the UK could tramways/light rail be installed?

I know that tramways exist in Birmingham, Sheffield, Blackpool, Manchester, London (Croydon), Nottingham and Edinburgh. I also know of proposals in Leeds, Bristol and Coventry, along with Cardiff metro under construction. However, we lag behind most of the EU on trams, where some countries like...

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