Bang for buck it's gotta be something from
Decathlon, or a calibre from Go Outdoors, or a good trawl of some classifieds. You'd get a decent hard tail for £500 used, with proper components and everything. Have a look on Singeltrackworld or even post a wanted ad. You could try pink bike or
eBay, it ain't rocket science, but the old buyer beware, it's easy to get stung.
Thing is, I hear you say, what is good componentry?
As a rule of thumb, for that money, used, I'd be looking for a hard tail, alloy or steel, with Rockshox, Fox or Xfusion fork, Shimano or SRAM chainset, (most likely Deore, Slx or xt cranks) pref Shimano brakes, and you'll find a mix of 9 or 10 speed at that budget. Don't get hung up on cassette or chainring wear, it's cheap enough to swap it for new, and you may even find some decent wheel sets on bikes at that budget.
I'll go and have a browse.