Some googling and looking on the student room forums will easily answer all of those questions. Well maybe not the first one, I don't recall seeing any statistics but for example if you were interested in fast jet statistically you'd have a higher chance in the RAF.
Maths need to be mostly SDT but very fast and efficient. Taking aptitude tests beforehand is pointless as none will emulate the official one. The only preparation you can do really is lots of maths, multitasking and maybe hand-eye -coordination etc playing games/using a joystick. The bottom line is that a tonne of preparation won't do much use if you really don't have the innate skill-set.
Does anyone have any up to date figures for RAF and RN aircrew recruitment? (just for my own interest). It's irrelevant though really you've either got it (and are willing to put in the effort) or you don't, don't let statistics decide.