I am only going to be in London for one day, getting in around 10:30 (er, 22:30) at night. I need a data only SIM so I can basically use GPS, Google Maps, Yelp, and maybe a little bit of browsing to find shops and such. So I want to pick up a Data SIM, that I may be able to reuse on my next visit.

At that hour most things are going to be closed. I know there are a few vending machine in LHR (I am coming into T3), and I did find a Tesco Express that is open 24 hours near Paddington. Which do you think I will have the best chance of finding a decent pay-s-you go SIM card (with Data)?