Odai,
I looked a BGS and was impressed with what I saw. However I went with CATS in the end, mainly because of the flexibility they offer (they are also substantially cheaper than BGS, particularly if you go for the on-line only course). After a quick look at the CATS notes I bought an old copy of BGS materials from eBay and have used these as the cornerstone of my study. In my opinion they are clearer than the CATS offering and it has been useful having reference to two sources of notes as having alternative explanations of concepts aids understanding.
You will find that the brush-ups really are just the icing on the cake and a hoop you are required to jump through by the CAA. In fact, CATS themselves advise that you don't attend a brush-up until you're consistently scoring 80% + in practise papers. Therefore I wouldn't give too much weight to the brush-up courses when choosing which school you go with.
The content of the course isn't that difficult. The people who seem to struggle with the course are the people who are not used to study, either because they haven't ever done any or because it's years since they left school/university. It undoubtedly takes quite a bit of self discipline - in my case getting up at 5:30am every morning, and working through to 8:30 before leaving for work. I've found that this has been enough to achieve decent (i.e. 90% +) results in the exams I have taken so far (fingers crossed for the results I'm currently awaiting
).
I'd also say that you should be at significant advantage coming from a scientific background. You'll probably know more than most of the tutors, and also you will have developed the ability to study independently during your degree which should be a big help.