If you would like to compete at agility, there are 2 competition bodies in the UK.
UK Agility (www.ukagility.com), you will need to register yourself as a handler and also each of your dogs. Once this is done you can enter a show in advance on their website, or you can turn up on the day and enter on the day, however it is more expensive. At your first show you must get your dog measured where you will receive a log book.
Kennel Club. If your dog is already registered with the Kennel Club due to their breeding you do not have to register, if not you need to register on the activity register, this is a paper copy method and can take a month. You will also need to ask for a KC log book for agility as this is needed for when you get your dog measured. Measuring is done at some shows and has to be booked, however I know of some that will be happening early 2011 where I can book you onto.