My tuppence.
You'll struggle to beat Scotland, but imo there are a couple of must dos down south. They're purpose built biking trails rather than all singing all dancing trail centres, but it's the riding you're going for, isn't it?
Stainburn, due to the technical nature of the trails. Don't let the fact that each trail is only a kilometre or two in length put you off…
Penmachno. great views, well built (albeit puddle strewn) trails that make you feel like you're out in the back of beyond, you'll not have the feeling that round the next corner you're going ot bump into a couple of hundred other riders. Closese thing I've ridden in these islands to 'natural' manmade.
I'd not bother with Sherwood Pines, it'll be a waste of a day.