There is a huge gulf in standards between Imperial, UCL, LSE, KCL, RHUL, SOAS, Courtauld and the other specialised UoL colleges and rest of the universities.
QMUL? Aside medicine and dentistry, not much to say about it really. Goldsmiths.....don't get me even started with this institution because even my head of department was quite critical of it when it came to choosing intercollegiate modules.
By your definition of "middle" - which I'm taking to essentially mean upper-middle, so the likes of Leicester, Leeds and Newcastle - I'd be inclined to say Royal Holloway, Queen Mary and City are all middle.
As the fifth-onwards best in their city, however, they're bound to seem inferior compared. And that is an effect that applies elsewhere - Nottingham Trent, Oxford Brookes, and Northumbria, along with Edinburgh's other Universities will probably always be in the shadow of their more prestigious counterparts to some, no matter how well they do.
Of the top of my head:
Top: Imperial, UCL, LSE, Kings
Middle: QM, City, RH
Crap: Greenwich, South Bank, London Met. LOL wasteman, ok I can't call everyone who goes to these unis wastemen because I do have friends at some of them (naturally- being black and all lol) but they're not good unis.
There are definitely 'middle' unis in London.
1. UCL, Imperial, KCL
2. Brunel, City, QMUL
3. Westminster, UEL, South Bank
It's the same in Birmingham. There's Aston Business School, Aston and Birmingham .... and then right down the other end of the scale, a right bunch of obnoxious morons called BCU.
Roehampton and kingston are amazing for certain subjects, but rubbish for others. Depends on what you want. So I guess they're 'in the middle'
Royal Holloway is not in London! I don't care!
But Queen Mary and Goldsmiths are both very good 'middle ground' unis.