I hate to refer you back there, but there's a dumpster diving subreddit.

And I don't know what the schedule in the UK is, but here in the States, people are moving out of their dorms for the summer. If the schedule is the same and you can get over to your local university or college or whatever, you may be able to score not just food, but other things that people just don't have the energy/ability to drag home.

A friend of mine used to offer end-of-termers $10 for their mini-fridges, clean them up in his shed over the summer, then sell them to next year's freshers for $40-50. Even if you don't have a car, you might find some small stuff you can clean up and resell.