I just finished my groundschool at BCFT, and found it to be great.
It's about the same price as Cabair, although the structure is slightly different.
Cabair do the exams in 3 sittings, one every 2 months, whereas BCFT have 2 sittings, one every 3 months.
This means taking 7 exams per sitting, where I think Cabair have 5,4,5 or something like that, but it also means only having to go to Gatwick twice, so only 2 lots of hotel fees, and only losing your motivation once. Trust me, after your first set of exams, getting back into study is really hard.
Also, Cabair require you to wear a pilot uniform, whereas BCFT is very casual, shorts in the summer, very relaxed atmosphere.
you only get 6 sittings to do the exams, so choosing BCFT means you have 4 left for any re-takes you may require. although hopefully you won't need any.
These are the reasons I chose BCFT. I don't think there's much to choose from in terms of instructors, both have some really great people on board, and are about the same price.