Greenlaning is public rights of way for vehicles in the countryside generally on unpaved roads. I.E gentle off roading (I say gentle but I assume some lanes are far from that but from what I have read it's\ a good place to start.
I got my Landy in July and, like you wanted to know more about Greenlaning and what it involved. I also wanted to find some people to go with so I could learn from them.
I started by getting a copy of Landrover Owners International Magazine and looking at the 4 pages dedicated to clubs in the UK. There are loads about.
Then I rang them or emailed them, chose the group I thought sounded most helpful and joined up.
I'm off on my first greenlaning expo in a fortnight - and am already having scary dreams about tipping my Landy over...
In particular, there is one based in Saffron Walden which should be in the right area for you. I have their telephone number but won't post it in public as I don't have his permission but if you send me a private message I'll send you the number.
Greenlaning, now there's a thing, be it meandering down pleasant leafy lanes in the home counties or taking on the deepest peat bogs in the yorkshire Dales, a well planned trip can be a joyfull if tireing experience. The problem is that in the UK you are about to become one of the most hated people on the planet, you will destroy preciouse wildlife habitats, cause damage to roads that are older than gods dog, pollute the ozone layer with your big throbbing diesel engine and destroy the peace and tranquility of the ancient english countryside. In short you are about to become a automotive piriah. May your god have mercy on your soul. Go for it, enjoy, and then go back and do it again.