To add my recent experience to the mix - Orange transferred me from pay-as-you-go to a monthly contract this week with absolutely no problems or down payments. As I have no credit history, I just needed to bring in a recent bank statement that had at least 5 transactions on it, and I walked out with a new (free) iPhone and a contract. I'm spending more every month than I normally would as PAYG, but if it puts me on the right track to establishing some credit, then it's worth it. Plus gives me something to do on my 40 minute bus commute to work =)
And on a banking note, I walked into a Lloyds a few days after arriving with my fiance visa and they had no hesitation about opening a currently account, even though I had no income. After doing a lot of research, they seem to be one of the only banks that will open a new resident an account for free. For anyone having trouble with getting a bank account, definitely check out their website.