Planning to buy a campervan in the UK and spend 6 months traveling UK, EU, Turkey and back to UK in 2014 - then more the next summer. We both have EU passports so no 90 day issues. What auto club would be best - neither the RAC or the AA in UK seem to have a lot of EU service.
ADAC (Germany) has affiliations with most of the other national autoclubs. I know you don't need to be a German citizen to become a member, but you may need a German residence or German-registered automobile... I'm not 100% sure.
ADAC is second largest in the world, second only to AAA in the US. But as Tom stated it's got partner agreements with pretty much every other AA in the world. With an ADAC membership you're never lost. They even offer special assistance packages in other countries so you get English or German speaking doctors, car repair men etc. And they even haul you and your vehicle back to Germany should you have a complete break down. So maybe your flights should also be from the US or Mexico to Frankfurt or Munich first, then on to the UK... Given the central location anyway you might want to base your travel plannings around Germany as your gateway country.