The rules have just changed. Your dog must be vetted not less than 24 and not more than 120 hours before departure. (I'm pretty sure this is the new regs, but someone will shoot me down if its wrong!)
Btw, I obviously don't know your plans, but on all our Euro-jaunts we use a site called La Chaumier, just outside St. Omer, where the owner's wife is receptionist at the local vet, and they will book an appoinment to suit your ferry.It's also a very pleasant little site.