I saw micro-SIMs in a vending machine right in LHR a couple years ago, so odds are good you could get one there. I happened to notice because I had just bought an iPad 2 right before the trip, which also uses the micro-SIM. I had done the research on taking it, but figured it would be too much trouble to find the proper SIM. Apparently not! They were labeled "for use with iPad 2" right on the package.