Viviki has a point when saying that it isnt as important where you go for the GDL. Not even magic cirle firms stipulate exact preference about where to go for the conversion(although some of the top firms insist on an institution that is recommended by the Law Society, ie. BPP, Nottingham, Col of L). My only remark there would be that if you go to BPP or College of Law for GDL, they guarantee a place there for the LPC (provided you pass the GDL).
BPP has an excellent reputation, but I turned it down after going to the open day. I felt it had a very business-focused attitude towards the teaching (probably something to do with the fact that it was originally a school for business and accountancy training), where they tell you exactly what they want doing, and you do it, no questions asked. It did seem to me that they were running a lawyer factory rather than an institution for learning.
HOWEVER, this is not to say that this is a bad thing. This no nonsense approach comes hand-in-hand with a reputation for getting people through their exams with very high marks. Considering the absurd levels of work that the GDL entails, this lean method is something that many students opt for.
Personally, I'm going for my GDL at the College of Law (London) instead. I just feel that the I'm better suited to their slightly more flexible approach to teaching. Not only that, but I don't have a TC yet, and I'm not pinning much hope on getting one this year, so the College of Law's impeccable career service will be of great use. But I have friends who went to BPP and they said, and I quote 'It's very good at what it does'. These friends have contracts with Slaughters. Make of that what you will.
Hope some of this helps.