As already mentioned above, I took the 441 bus which passes via T5 to the Tesco next to Ashford Hospital (I think that's the bus stop name). It's a reasonably large store and you should find a good selection of Angel Delight in there.

The 441 bus costs around £4 return IIRC. It doesn't run that frequently, every 20-30 minutes so it is worth checking out the bus times.