By law a vet cannot refuse to issue you with a written prescription if you request one. What they can do is charge you a suitable fee for their time in producing a prescription. This varies from practice to practice.
Before going down that line can I suggest you ask them about a couple of alternative products? There is one product that has been available for a few months called Propentofylline and another just about to be launched called Vitofyllin. Both these products are exactly the same and you may find your vet can do a better deal on them.
We can all be guilty of replacing like for like with our stock and our computer actually tells us when something needs replacing, it needs a human to step in and compare the products to see if these generics may be cheaper.
Do remember that your vet is in business like you. If everybody goes and gets prescriptions you will see fees rise a lot. By contributing a little each time you take some drugs/flea treatment/wormer you keep the prices manageable. Think how you feel when customer insists on supplying the cheap, nasty B&Q special consumer unit for you to fit.....