[fivefifty]

The Harrys Farm youtube channel explains it fairly basically in this video, I'll try and link direct to about where he talks about it specifically (although whole thing might be worth watching if you're interested in how farming works in the UK) https://youtu.be/b6m29rSSmCM?t=552

For those who can't watch the video, basically the gist is most of the food in shortage (lettuce, tomato, cucumbers etc) is normally grown in the UK in large greenhouses from local producers, but energy prices got too high last year making it uneconomical to grow in a greenhouse as obviously it's energy intensive to do so. The govt refused any support and supermarkets rejected the potential higher prices and instead decided they could just source product overseas instead. The greenhouse producers therefore didn't plant much for this season, but the foreign sourced produce also had a poor harvest which resulted in no excess available for export to the UK and therefore some stores now can't stock some fresh produce. Energy prices have come down and so local UK produce growers are now growing again and should be back to normal supply in a couple of months. Although he mentions earlier in the video that this years wheat crop is doing poorly and that is normally used as animal feed which could mean potential future shortages for things like meat and eggs.