Kapo,
Actually I was refering to this material here:
https://www.bristol.gs/product/compl...onversion-kit/
The cost is £800 (around 1000USD) plus shipping, but it's definitely not necessary to use it. This was just my option as I'd like to have the CBT with me too, but you can buy only the access to the Bristol question bank as well, which is way cheaper.
Either Aviation Exam Question Bank or the BGS Question Bank are great. I just don't recommend using only these as a source of study, because they're simple question banks. Memorizing the questions/answers might help on some subjects, but chances of failure are high. I think it's easier to have a full understanding of the syllabus. You'd better have some source of theory reference as well. Paco's book is great and much better and easier to read than the tiring CAE/Oxford books. As he mentioned above, the .pdf version costs just $59,95. I bought the paper version because I don't like reading long texts in the computer/tablet/phone. The price of the paper version with shipping was 110usd.
As for the validity of the exams credits, I think Paco has already answered above.
I have similar flying experience as yours. 3100TT with 1600h on the 737 as a F/O. The UK CAA just asked me to send a copy of my passport, my licence, my ratings, my medical and to fill a form called SRG1192 (you can download it on their website). About 5 days later they approved my exempt from training status and I was able to start booking my exams.