Wouldn't touch imperial the open day was enough to put me off for life it felt like I was stuck in one of those Open Univeristy type Science videos they make you watch in school the play had a boy to girl ratio of like 10:1 and a uber geek to normal person ratio of about 100:1
Bristol is a nice city and the University has a great rep, it's a bit pricey not as bad as London but you get no extras to help out so that is definately a consideration. Had a mate who went there and I have to say a lot of the people he introduced us to were a little Rah, the jolly hockey sticks brigade if you know what I mean. Of course that was probably just the people he was hanging out with and he did walk into an awsome job straight after graduating.
Sheffield, well I live here so I'm going to be nice about it! the city is fantastic and what I have seen of the student life up here is pretty awsome. It's definately cheaper than the other two Options and Sheffield is a proper Uni with a good reputation, plus you get to pray on the girls at Hallam!
Choice is yours but go where you will be happiest ignore job considerations as a 2:1 from any of those Uni's in engineering type stuff will get you a good job and three years is a long time to be miserable especially as they are supposed to be the best years of your life!