In Scotland, NHS veneers have no guarantee due to the very high possibility that they fracture. If they warned you of that chance before doing the procedure, they would have no obligation to replace it or fund the replacement, at least as far as the NHS board would be concerned. However, unless they have documented that in your records they wouldn't be in a good position.

I'd recommend you ask for their complaints procedure and write a letter/email complaint. They'll take that much more seriously than a phone call, and unless they're insane they will speak to their defence union who will tell them to refund you. Just explain in the letter in polite tone the issue you have had and what your solution would be: a refund to allow you to have it fixed elsewhere. I don't know many dentists who wouldn't jump at that opportunity to make the complaint go away.

Oh, also models for veneers aren't routinely stored. They are usually thrown away. The only models I'm obliged to store are for cases involving bridges, so it's not surprising to me that they don't have them.