Mountain Bike Trials and Events
www.moredirt.com offers information from Plymouth to Ipswich to Birmingham, Manchester to Newcastle Upon Tyne about mountain bike and skate boarding venues.
www.b1ke.com operates several parks in the SOUTH for example Tidworth and Longleat.
For UK wide routes try:
Sustrans enter “mountains” in search.
https://www.sustrans.org.uk/search?searchTerm=mountains
“Whether you are a seasoned mountain biker or have just bought your first mountain bike, Cycling UK has the advice, training, courses and support you need to ride off-road.”
There is a Mountain Bike magazine.
https://www.mbr.co.uk/mountain-bike-routes
And a sight where you have to pay for information.
https://www.mbuk.com/trail-guide/
Trail Fork with numerous routes.
https://www.trailforks.com/region/united-kingdom/?activitytype