Originally Posted by
diburning
Doing my own little bit of Googling, it looks like the best plan is that if it can wait until you leave the airport, then wait until you leave the airport. You can either buy an overpriced sim from the vending machine, or buy an overpriced (and possibly even worse value) sim from the Sim Local store (if you're at Heathrow)
W.H. Smith sells 3 and other SIMs. The 3 SIM cost me GBP 0.99.