Hi,
There are previous threads on this topic, you may want to look back. I'm not local to Brighton, so cannot answer those specific questions. However here are answers to your generic questions:
* Do my introductory hours count towards my PPL flying hours? YES. All flying training does, even if it was several years ago, as long as you have it logged.
* How possible is it to study & take the exams independently and just pay for the actual flight training? YES. Most people, in my experience, study at home for the ground exams using one of the various series of textbooks which are available. I certainly did. Some flight training organisations offer ground school but I managed to pass the exams perfectly well, along with my fellow students, on self-study alone. I then sat the exams at my school (from memory it was £38 per paper).
* Lots of people have said pay as you go rather than upfront for training - I'm not sure exactly what that means - can anyone explain? YES. Schools often offer discounts if you buy "blocks" of training in advance, rather than paying lesson by lesson (many seem to offer 45 hour blocks for example). However, given the hand to mouth existence of many flying schools, I don't recommend signing up for blocks of more than 10 hours at a time. If the school goes bust, you would need a miracle to get any money back.
Good luck, it's a tough journey but very rewarding!
Glorygal