Regarding question banks, I used AE but tried out BGS for comparison for a week prior.
They are very similar but I found AE has more questions, has already implemented the new question format (dropdown boxes with text selection, tick multiple boxes, enter number for answer). I personally found the user interface of AE better, especially the retest wrong answers, sync with offline iPad/iPhone apps. Explanations are generally quite good but some are lengthy and/or not tailored specifically to individual questions. You only see these if you click on the tab, such as when you get stuck or pick the wrong answer. I found this a good way to deal with questions as I progressed.
BGS seemed to have a slightly smaller question bank, which may be an advantage if this is because it has dropped the more obtuse questions that don't appear. Extensive search and filter functions. I only used this on a computer and found it displayed a brief and helpful explanation after you selected an answer, no choice to look at this first.
With EASA adding thousands of new questions to the QB annually, I'd expect the online QBs to diverge more from what you actually get in a test. They do prepare you, especially on exam/answering technique, and reinforce the key learning points, so remain an essential tool in the ATPL TK learning process.
Regarding price
UK CAA exam fees went up in April 2017 to £69 per exam.
Guidance states that some of the UK CAA foreign exam centres may charge more.