I've bought a Virgin phone and am on their non-debit pay-as-you-go plan. (You have to have a UK bank acct to do the direct debit--they won't do it from a credit card, and we don't have an acct yet.) The tarriff is quite reasonable in terms of PAYG plans; only t-mobile seems like it may be a bit less but their tarriff is so confusing that I don't want to mess with it.
I also have to say that dealing with Virgin Mobile in the UK has been one of the best experiences I've had in terms of customer service. Get this--when trying to sort out a problem with them, they will often volunteer to *call you back.* That's absolutely unheard of in the US. There have been a few minor problems, but they have been more than willing to figure them out and usually gave me extra call credit for the trouble. I really recommend them.
And it should be noted that Virgin Mobile in the US is just a reseller marketing name for another company (I think it's Sprint, but I'm not 100% sure). The regulations and the networks and everything are totally different from the Virgin Mobile in the UK. I understand too that Virgin Mobile in the UK is a reseller of t-mobile, or may have started out that way. The PAYG minutes don't expire unless you don't make a call for 180 days (though the customer service people said it was one full year).