Cycling from London to Croatia?

Hi there, me and 4 friends are planning to cycle to Croatia this summer. I was wandering if anyone has previous experience with long distance cycles, and give us any advice, for example how many mile/kms we should aim to do a day. We're still trying to decide the bike we are going to need. Hopefully we should have a support vehicle so won't need to carry to much. Is there any really good places to visit along the way? thanks a lot