Just happened to see this thread still active. I'm back from my trip and found my own answer. I had a longer than expected layover at Heathrow (Purple coats did not know what they were doing) and checked out Dixon's Travel. They had O2 SIM cards for 99p and I loaded it with £10 for 100 min, 1000 text, and 2GB data. More than enough for a week. Poped it in, called activation number and it worked like a charm immediately.