Ah, I see. Sorry I assumed you were already in the UK. I'll try to help. Stereotypically, the sort of person each institution you mention attracts is quite different (in my experience). Disclaimer: I've got nothing personally to do with any of the three places you mention.
Durham tends to attract a lot of well spoke, affluent, highly capable, more independently minded students from supportive backgrounds. Durham is a traditional, well respected, solid, university. When people imagine the feel of a British university, Durham isn't far off (also consider Exeter, Bristol, Cambridge, St Andrews). Durham produces leaders rather than workers. It is in a well balanced area with plenty of air and space to breathe and relax.
Warwick for the past 10 years has been expanding from well, relative obscurity. They're pretty cosy with the Labour party and have benefited (some would say unduly) from them being in government since 1997. Warwick is a much newer (late 1960s?) university than Durham and they have a very high opinion of themselves, but never seem to actually do a great deal. It's worth noting that university isn't in Warwick, but closer to Coventry. The typical Warwick student is someone from a slightly less affluent background who has come from a modern 'comprehensive' school and learned how to pass exams but not think creatively. Warwick produces workers not leaders.
Imperial is the best well known of the three you mention, they're a good, soild, old institution with an excellent international reputation, the best of the three. The only downside is that they're in London (London prices, London pollution, London noise, London traffic) which really puts me off. A lot of international students are attracted to Imperial primarily because it's a well respected place and is in London which is somewhere they've heard of. It's also close to Heathrow for when you're flying in from/out to the States.
Hope I haven't been too blunt describing Durham and Warwick, just trying to be candid.
It all depends on your personality and what experience that you want, I think.