If it was me I would 1. Make 100% sure of my preferred vehicle. 2. Present the government guidelines, my entitlement and VAT exemption form (if appropriate at this point of order), and ask for a quote sans VAT. 3. If they refuse to quote me/serve me, tell them I will be composing to the head UK office of disability discrimination and leave. 4. Email the dealership management/complaints manager the details of the exchange thus far – and copies of guidelines and paperwork – and that I am providing written notice of the escalation of my complaint to UK head office of said company, and taking legal advice if necessary, if they cannot provide a satisfactory resolution within one working week. 5. If I get no VAT-free quote, follow through on all of the above.
That’s speaking personally. If you have a legal right, for the most part, pressing it comes out in your favour.