I suppose it depends on your definition of decent.
I bought a Boardman CX Comp for £650 brand new a couple of years ago and it’s been ridden a handfull of times only.
It’s a good bike but the trouble is most of this forum is likely spoilt by the mtb quality they already have.
The Boardman gears are clunky, the brakes are horrible, it weighs a ton and it doesn’t really do anything especially well.
For long road rides with the occasional hop over a field or bridleway then probably ok but anything more off road based a decent hard tail is faster and more comfortable.
The Boardman Comp is probably one of the main contenders for the OP’s budget, but will only really fall below £500 in the sales. I managed to get a CX Team for £600 in the sales + BC discount. Although I have to say, while it gets used it a lot, it hasn’t been the most inspiring bike for me. It’s not light, it’s not spritely, and just feels kinda dead. And yeah, although I quite like the function of the Sram gears, the shifters can be hard work.
Can’t really expect a whole lot for £500 though. And the Boardman is a great bike for putting some miles in really. I’m sure the Comp isn’t that far behind.