I use World Nomads, they’re a backpacking provider really, but they’re by far the cheapest, you just need to add in the correct additional cover for MTBing. I’ve had to claim from them once, non-medical thing and they were fine. Worth noting, they’re very open and honest about cover when pished – basically if you fall down the stairs after 18 pints, don’t bother calling them. Lots of insurance cos have similar exclusions, but they’re not so open about it.
Dogtag are also good, but cost 3x as much, I had to claim from them too, cancellation due to accident. They were slower, but okay.
I’ve used the Post Office before now, it was cheap, they cover Mountain Biking (not racing) as standard, but I felt a bit nervy about not having specific cover, but I asked the question on their live-chat thing giving a full explanation of what I was doing and they were fine with it.
I should add I have ‘normal’ travel insurance too for normal stuff.