Airquiz is excellent, the tricky questions have explanations attached with images, graphs, etc. Another good reason to signup is that it is constantly updated - I remember ordering the JP books brand new because I wanted to be sure everything was up to date, especially law. Well, turns out the 2010 edition was still missing AIC revisions from early 2009
Compared with CAA website, airquiz had all the correct figures. Just do the tests over and over again until you consistently score above 90. Only one caveat, the language used in the questions is different from what you'll find in the papers.
You should also get the "JAA PPL Exam Secrets Guides", you can find them from a number of sources. They only exist for Law, Aircraft, HPL and Met for now as it's a new series. The questions there are exactly like the ones in the CAA papers, word per word. Well worth the money.
Finally, get the TSA fingerprints done in the UK. They tell you it takes 2-3 days, but I wasted 10 perfectly flyable days in Florida waiting for TSA clearance. I know it's expensive, but so is changing your return flight at short notice.