Costco were doing a Muddy Fox Mayhem for £200 which actually wasn’t bad. Reasonable components and mechanical disc brakes. Built one for a customer.
Only came in 19″ I think.
Think they still had them last time I looked.
A friend got a decent enough Carrera MTB at Halfords reduced down to £250 which was also good value.
Apart from that check out Gumtree or other local classifieds. Look for a bike that someone’s bought to “get fit” on or got it through the RTW Scheme and then lost interest.
If a fat person answers the door you know you’re on to a winner coz the bike will be in “as new” condition 😆