Been off riding so late response 😉
Thetford forest is actually more technical than Dalby red but that doesn't preclude that Dalby red (for me) was one of the worst trails I have ever ridden.
Why did I go there? because I have a car and like to see lots of different places and not just ride the same old route week in, week out.
Whats good in between Suffolk and South Yorks? For me Peaks every time, lots of different routes all natural and very challenging in parts. Trail centre wise I've only ever done Sherwood Pines and Cannock Chase and both are very much like Thetford ie flat and technical single track in parts, neither that I would rush back to do again as too similar to what I've got on my doorstep so I avoid in preference for Welsh and Scottish trail centres.
BTW Cambridgeshire's that flatest place in UK, Norfolk a close 2nd and then Suffolk as where I live there's a hill I use for training that they honestly use for hill climbing competitions! hehe