OraNurse is said to be highly recommended, you might want to try that?
Not many GPs will prescribe toothpaste as that is usually a dentist's area of expertise. Also it costs the NHS silly money to prescribe certain items so they should be cutting back, bit like prescribing paracetamol that you can buy for like 12p. I'd suggest just looking at toothpaste on Holland and Barretts that's likely the best place to start and you can read people's reviews.
I missed your comment as I jumped into my notifications for this discussion late. Nonetheless, thank you