Check out Charity Adventure:

http://charityadventure.org.uk/

I cycled from York to Amsterdam with them last year with a womens only group - my first charity ride and the experience that really got me hooked on cyling. It was fantastic, the guys don't do it for profit and are amazingly encouraging and helpful. They will get your to your destination even if it means they have to push you there! I'm doing Amsterdame again this year (see if we can improve on the money raised last year which was about £38,00), but there are quite a few other rides covering different levels of ability too, so if you fancy a go, have a look!!!!