firstly, if possible, do the LXL A-Levels.. I did cambridge and was my A's were considered equivalent to those who got A's for LXL, but LXL is WAYYYY easier.

1) Depends how good you are, I started studying ~3 weeks before the exams. some people started 3 months before and couldn't get straight A's

2)just download the entrance requirements from any university. some courses will specify specific subjects, others will just specify a number of A's.

3) only cambridge (as far as I know) requires more than 3 subjects to get in. The most important thing to do is pick which three subjects to take. Oh, avoid further maths. Not many people get A's

LXL = edexcel. sorry, my friends who took it call it LXL. dunno why I took OCR, so I got annoyed when I saw their syllabus. Most of my first year uni maths course was covered in OCR but not in edexcel