I get mine (2000IU & K2) from Amazon where 400 tablets are between £8 and £10.

That is before any bulk discounts are considered.

VitD as a preventative would probably be distributed through the GP system rather than the hospital system, and so be procured locally and subject to the Drug Tariff system.

I think it would be great for this to be procured centrally for great discounts and dispensed locally exempt from our tariff guides and prescription charges. After all, the NHS has great clout as it employs 1.5M people and we spend > £300B on healthcare. But the NHS changes very slowly, mostly because it’s the ultimate political football and suggesting any change means the usual suspects screaming that you’re trying to destroy it. We’ll get to preventative medicine when we’ve tried everything else.