I had an interview at their Porton Down site a few years ago - as has been said before, expect it to be a professional job-like atmosphere, probably without the social perks of a University (hundreds of clubs, a Union building etc). However, most people working there seemed to be quite young, I suppose they've had a big recruitment push of graduates etc.
You'll also still be affiliated to a University so you might not lose all the perks of academia.