It offered a variety of SIM cards, including a data pack from 3 UK that offered 1gb of data for a month for 20 GBP. I got one, stuck in my wifi hotspot, and had Skype up and running on my iPhone to coordinate airport pickup within less than a minute.
We had the Internets some 15 minutes after landing. That's the way it should always be.
P.S. It took us much longer to locate the toilets. You won't hear too many superlatives concerning Heathrow from me.
Image by whatleydude, Creative Commons license