It all depends on your chosen path to the PPL. I went to the States to get my PPL in four weeks, and I knew there was no chance for me personally to study and pass my exams in that timeframe (although there were other , younger, guys on the course who did). So I started studying about six months before I left the UK, and took five of the seven theory tests here (I decided that the Met and NAV exams would be best taken when I had more practical experience). I also did a few hours flight training in the UK to help understand the theory with some practical work.
I agree about
Airquiz, it proved invaluable in self testing (and as you get two years membership, I also used it for my IMC training a year after my PPL).
The order to take the exams is an interesting question. Perceived wisdom when I did them was that Human Factors was the simplest. However, this subject has the least number of questions, which appears easiest on the face of it, but it also means it is the one with the least number of wrong answers you can afford, to acheive the pass rate.
Good Luck