The cost of a healthy eye certificate is priceless! (Well it is in my breed!).
Seriously, here is a link to the BVA site, which gives full details. You may get it done cheaper if you can find a breed club putting on a testing session, or sometimes at dog shows they have sessions. Weekly dog papers (Our Dogs/Dog World) list forthcoming sessions at shows.
Let me know where/when you want it doing and I will see if I can find one for you. The reason your vet cannot do it is that it has to be done by a specialist opthamologist, who is on a panel selected annually by the BVA.
Depending on what conditions are relevant in your breed, a yearly examination is recommended in most breeds. Some conditions are certified for life though, so only need doing once. Others are present at birth, and do not get any worse, some get worse as the dog gets older. The Kennel Club site gives details of which breeds are susceptible to which conditions.
http://www.bva.co.uk/public/chs/eye_scheme.asp