Northeast of Carlisle, the Kielder Forest area is part of the Border Forest Park just west of the Northumberland National Park. There are two nice mountain bike rides: Deadwater Fell Summit and Kielder Water Loop.
The Deadwater Fell Summit is an 8 mile round trip out & back ascent of the summit with 300 m. vertical. The route begins and ends at Kielder Castle.
A good starting point is near Kielder Castle on the northern end of Kielder water.
Overall, Deadwater Fell Summit is the 33rd most popular mountain bike trail of all 93 mountain biking rides in UK.