As already stated here it's the reputation of the department and the supervisor that counts. Check the RAE (research assessment exercise) rating of the department - most departments publish this on their web site, but you can also check on the HERO web site. The most recent RAE took place in 2001.
The larger universities do tend to have more high rated departments, but working in a bad lab in a good university will do you no good. At this point it will be difficult for you to judge the quality of your potential new supervisors, but the research direction they set for you can dramatically affect the progress of your PhD. A good PhD project should have achievable research aims as well as providing a genuine contribution to knowledge. In general successful supervisors have successful students, so talk to as many people as possible when you go for interview or to an open day. If your supervisor does not have a realistic grasp of what you can achieve, it won't matter how hard you work.