Was the stuff at Halisham all green lanes? I’ve never heard of it but it’s not far from my parents so might pop in oneday.
Regarding routes, the trailwise website is one of the best I know (just be warned that not all routes marked are up to date and some have restrictions in place).
I do all my greenlaning around Dorking. I’m based in SE London I can ride there - just take the back roads and it takes about an hour. Surrey’s got enough lanes to give you a full day out. Am finding it more and more tedious lately though with the dog walkers 'n horse riders - some are happy, some are grumpy, and it gets worse in summer 
I’m mostly tempted by enduros but I need a van/car+trailer first