GDL providers are all similar in terms of course delivery, as the course is very standard. When people say BPP, CoL, and Nottingham Trent are best for the GDL, I think they have perhaps become a little confused along the way. This is in fact largely with reference to the LPC, where there is some form of hierarchy. The same cannot be said to exist for GDL providers.
BPP and CoL may in fact NOT be the best places to do the GDL if you are looking to go on to do an academic LLM afterwards. The courses are often said to be much more spoon-fed than at other places, and as such don't develop any further academic legal skills, or at least to a lesser degree than at some universities.
If you would like to do an LLM afterwards and would like to consider doing the LPC at a reasonable university, perhaps looking at the GDL offering at the University of Birmingham, the University of East Anglia, Keele University, Swansea University, and Oxford Brookes University.