I know they are terrible. But a trawl on gumtree get not much for £75 second hand. The best at a low price I have found new is the rockrider 300 from decathlon at £119. Nice and safe with no suspension. Difficult challenge but what’s best for the least price?

Halfords own Carrera brand, hard to beat for the price

Even the bottom end decathlon bikes are fine- done a good few hundred miles on the hundred euro rigid ones over a few summers of leading trips. They are heavy and stiff (i think the frame is solid, not tubes), but there’s nothing on there that won’t last a reasonable while. Unless you’re maybe 15st or planning on seriously abusing it, in which case budget for a new rear wheel as you may bend the axle.

Depending on where you live you may have a recyke a bike locally. Ours always has something worthwhile at the 100 to 150 mark.