How do people actually dumpster dive to get free food? Are there any other cheap/free ways like this to get food?

My local food bank can only provide 8 packages with referrals every time before you run out, and I have, but my situation hasn't improved financially due to various set backs and I'm struggling to feed myself. I've heard that supermarkets throw out massive amounts, but have never been in a position where dumpster diving seemed feasible. People who do it, what time of day do you do it and how do you find good spots? UK resident for ref