Hi Rebeccajo
Yes, as already stated, you'll have a zero credit rating for the UK at this stage. You'll need to start to build up credit once you are here. Have a good search on this section, there's actually quite a lot of posts and threads covering this subject and also good ideas on how to actually go about getting that credit rating started - first being getting a pre paid or pay as you go mobile or even paying up front a deposit for a contract phone and taking it from there, alongside perhaps getting a very low amount credit card with the bank you join - say a £250 limit to start with.
If you have an American Express credit card in the USA, they will let you transfer the card to the UK operation, that will instantly be a 'shot in the arm' for a UK credit reference agency to start giving you a score. This is done internally at AMEX, and if you keep your payments back to them regular and good each month as soon as you arrive/trnasfer the account, the quicker the UK agencies will start to increase your credit rating here.
Good luck!
Cheers, DtM! West London & Slough UK!