That's easy then, the UK is one of my areas of expertise
Do you hack out already? This is the key to building up fitness, experience and confidence for both you and your horse. Explore all the local bridleways, find out what National Parks, Forests etc is available to you by trailering out. You need to have experience in trailering to a ride too.
Do Fun Rides. I don't know where you are but most rural parts of the UK are full of groups, hunts, or charities organising fun rides. Normally you'll be laying about £10 to enter for 4, 6, or 8 mile rides.
Then look at the websites of Endurance GB and ummm I can't remember the other one. Both endurance organisations organise rides that also have non-competitive rides on the same day. It's an easy way to learn the rules without the angst of competing.
Oh, and check out the UK TREC organisation. They do a lot of useful stuff.
Hi hope that gives you enough to be going on with!