Give everyone going for a flu vaccination a bolus dose of vitamin D3 at the same time, and a tub of a maintaining dose of tablets to take away... a high-ish dose of around 4000 iu, rather than the standard RDA of 400, which is merely enough to prevent people dropping dead from rickets/osteomalacia... the bones get first dibs on vitamin D, so you need to take higher amounts for the immunological benefits to kick in. Obviously people with conditions where vit D is contraindicated would get a much lower dose/none as appropriate.
Strongly encourage the rest of the population to take around 3000 - 4000 iu of vit D3 daily. The government would have to facilitate large-scale manufacture/import and distribution, as a sudden surge in demand would cause shortages and unavailability of the product.
Make available vitamin D testing at all larger pharmacies for a modest cost, say £5 - 10, and make available a guidance chart with advice on how much to take.
The only way out of this is through, and the main limiting factor IMHO is widespread vitamin D deficiency/insufficiency. Many studies are showing a close correlation between vit D levels and risk of dying/hospitalisation from covid. If we could get the average levels up to > 100 nmol/l across the population (ideal level is c. 135 nmol/l according to a recent Israeli study), almost all of us would get out of this alive, and with very little impact on the economy, barring the few days that people need to take off work with a + test. It would also have huge positive benefits for many other diseases, and has the potential to save thousands of pounds per capita across the healthcare system and the huge impact on the wider economy caused by lockdowns.