I think expansys.com also sells prepaid cards for good prices compared to the internet sim card sales places. But both LHR and LGW have The Link stores inside the airport that sell starter kits for Virgin Mobile, Vodafone and I assume the others as well. Something I found out today is that the Virgin store in the duty free area of LGW sells Top up cards for duty free prices. They are about 15% off face value as I recall (its been a long day).