I agree with HWD, I used to work in rented offices in the tower and used BBA to keep my PPL current (after getting it revalidated through them).
The aircraft in general were good, I pretty much had complete access to the Tomahawk as no-one else wanted to fly it! and the instructors and resident examiner were a good bunch of guys both in their professionalism and general manner. Had many a laugh with them in Biggles Bistro over lunch. I think their organisational skills has improved a lot with regards to flying slots and ground instruction.
The cost unfortunately is standard for anywhere around London, if anything I found them to be on the cheaper side of the options.
Though I had the added advantage of already being at the airfield so travelling costs were negligible.
CH
By the way in the general aviation magazines someone offers a C-152 for hire at Biggin that seems cheap, though it might be difficult to actually get the aircraft for a flying slot.