Hi and welcome to the forum.

OK, the credit situation varies a bit from country to country. But basically your credit record in the US is lost for the ages once you make the move. The UK system is probably the most like the US - credit reports, etc. - but AFAIK it's based solely on your UK experience. When you first arrive, you can sometimes use a "letter of introduction" from your bank in the US to help establish initial credit lines.

On the continent, credit is a rather different beast. In France, for example, the banks are pretty strictly regulated in terms of what sort of loans they can make to customers. Credit cards function more like what you know as "debit cards" in that you're limited to the balance in your account and there isn't the option of rolling over charges to the next month. It's a pain at first to get used to, but boy it keeps you out of the sort of trouble so many Americans seem to be in lately!
Cheers,
Bev