I went with CATS mainly based on price, location, reputation and their online training course (it's also split into 3 modules)
I have heard BGS has a very good reputation, but found their location was miles away from me and the price was more than what i wanted to pay.
These are the main two which i've heard good reports about. There is GCNS at Glasgow and GTServ at Bournmouth, Cabair at Bournmouth and Oxford for you to choose from.
At the end of the day its a choice you have to make yourself, but bear in mind the things ive mentioned. Location was an important one for me if you attend the brush up courses.