http://shop.tescomobile.com/mobile-phones/pay-as-you-go#price-min=0&price-max=1000&sort-by=phone-price-low
(* for those unfamiliar with the UK, Tesco is the largest supermarket chain)
This should be just fine. Any chance that there is a Tesco at Heathrow?
Nope, just the SIM vending machines near the exits from customs, the
SIM Local store in Terminal 1, and the Vodafone store in Terminal 5.
For what you want, any SIM should work OK. Remember that incoming
calls are paid by the caller (not really "free"), so the only
difference is the outgoing cost. I doubt you will make �10 of calls
during the week no matter what carrier or plan you have. I've never
seen them more than 20p/min, so that's at least 50 min of outgoing
calls.
R's,
John