I'd like to agree with 'I just want to fly'...being yourself is the best strategy!
I've been flying with easyjet for six month now and I can remember my assessment day very well....and being nervous is certainly part of it! But remember that they allow a little bit for it as well. And after the first few group exercices you'll forget and relax anyway. And don't forget, if you don't make it through it might be as well not the right company/job for you anyway. And if you're determind you'll try again!
Anyway, from my experience it's a long and tiresome day but lot's of fun as well -just try to enjoy it.
And if you are at all flexible with your location put Bristol on the list -they are hiring for Bristol like crazy, so it might help a bit!
Good luck to all of you!