If you want to do affiliated classes then you need to be registered with the SHB. http://www.sporthorsegb.co.uk/
They do novice and open classes which range from around 3'3 to 3'9.
If you just want to do local or unaffiliated, then you do not need to be a member of any society, and the classes can be anything from 2' - 3'6".
There are quite a few working hunter posts on the forum which go into more detail, but there are several people on here who do workers and will be able to help you if you have a specific question.
if its a horse, then you will jump a course of natural fences of about 8 to 12 fences. then you will usually do a show, where you are required to show walk, trot, canter on both reins, and usually then you gallop. the judge will ride the horses, and then the judge may ask for the horse to be trotted up (take the saddle and boots off and a martingale if used). then you will be placed!
if its a pony it is the same but the judge does not ride the pony.
the heights differ from show to show generally and the novice horse class usually ranges from 70cm to 90cm, with the height of the fences in the open increasing with the age of the horse.
you wear a tweed jacket, and long boots for anything over 14hh. the tack can be brown or black, and you must have a snaffle bit for novice classes. the horse boots shoud be preferably brown or black. good luck!