
check optigen for the clinics - they're all on their website. You contact the clinic and if it's nearby they'll take the blood and send it off, if you're doing it by post you have to go to a vet for them to take blood which you send to the clinic and they post off together. You get 20% discount, plus more discount for paying by credit card.
The clinic gives you a reference number to quote and you have to register with optigen prior to the clinic day, give all your dog's details (so you need registration, microchip parents, eye test dates or whatever is relevant) then you give your credit card details and print off the relevant documents (for your vet to sign and to go with the blood sample). If you give them your email address they'll confirm they've received it and then give you the result quickly by email. They don't take money out until they get the blood sample.
There are quite a few clinics around the UK and if you ask them I'm sure they'll take a postal submission - if not you can send off just the one sample, but you wont get the discount.