A prescription for GF pasta? Is it any different from the rice and potato based stuff sold without prescription at Tescos?

Yes; this stuff is made by removing the gluten from wheat. So it's not safe for people with a wheat intolerance rather than a gluten intolerance, but OTOH it's generally superior pasta – better taste and texture.

I have no idea why it's only available on prescription. I can see the value in GF staple foods being available on prescription, because that means people who can't afford to buy expensive specialist food can still follow their GF diet if they can get themselves into a category of people who get their prescriptions free. But there aren't any legally controlled drugs in it as far as I know, so I can't see why it shouldn't also be sold in the shops for those of us who find faffing with prescriptions to be a huge annoyance. But, for some reason, it isn't.

Gluten free people can eat the wheat free pasta though, can't they? In which case it's probably not worth the supermarket's while stocking two things for a very small market, even if that means they stock the less nice thing that serves a larger group of people.

Perhaps; but even if supermarkets didn't feel like stocking the stuff, I'd expect there to be specialist shops, or mail order, or some means of getting hold of it, unless the manufacturers were deliberately refusing to sell it on a non-prescription basis.