This is a huge subject and not straightforward!
Ideally grow your own. If not, then shop locally, using shops that also source their produce locally e.g. butcher.
Use local box schemes for fruit and veg.
You may need to ask where food was grown/produced - the twice weekly market may sell food from outside UK because it is cheaper.
Buy food in season: tasteless, flabby strawberries imported in December are not a patch on British ones in season.
Visit pick-your-own farms or farm shops.
If you cannot buy products grown locally, e.g. coffee and tea, then opt for Fairtrade.
(We would welcome any more ideas, input, examples.)