My regular travel insurance I use for skiing and stuff (Travelinsurance.co.uk which is a rebrand of InsureAndGo), excludes MTB holidays (only noticed that this year) and also excludes racing and riding on “extreme ground conditions”. Digging around this is a general get out clause and typically means anything other than tarmac. Can notify them (for a premium) if doing proper MTB but it then says it invalidates the personal accident and liability cover for the duration of the activity (seems kind of pointless really).
In short, standard travel insurance likely won’t cover proper MTB.
I’ve used Snowcard for specific trips however. One trip policy. I’ve looked at annual insurance with them for MTB and skiing (I ski in US a lot, so has to be worldwide), but it’s stupidly expensive. Cheaper with a standard insurance and then one off insurance for specific trips.
Also with Snowcard I dial down stuff for baggage, money and anything non riding/bike related. Leave that to my regular insurance for lost luggage, delays, theft etc. Snowcard would cover bike related injuries, recovery and the bike itself (although though last I looked only covered to £3k for the bike).