Normally in the US, vent disease is usually the first thing that rabbit-savvy vets will treat for in the case of a swollen and/or scabby vent area (often with scabs on the face and paws). There is no testing done, they just give them the shot and if it doesn't clear up, then look elsewhere.
In the UK, however, there is the possibility Myxomatosis, which can have somewhat similar symptoms, so testing is advised.
I don't know about the UK, but on this side of the water there is a strong movement underway to stop rabbit sales in pet stores.
I'd suggest going to the heart of the matter and using this example of uneducated staff, inadequate vet care (for rabbits), etc. as a rallying point.
Hopefully others will have some specific contacts for this.
You may also want to try rabbitrehome/rabbits united at
http://forums.rabbitrehome.org.uk. They're a great source with an awesome forum.
Poor bunny.
Thanks for caring.
sas :tears2: