UK high streets have been in some kind of decline (IMO) for decades now. Long before online shopping, COVID and its repercussions, and the current wave of street crime (phones, watches) and front businesses (vape and phone repair shops hiring dozens of "skilled" workers on visas... to work in a tiny cupboard unit with implausible turnover). Even when a new "luxury" apartment block is constructed, the retail units often remain vacant for years before being occupied by a constantly changing list of tenants, none of which seem to be able to make a success of it. The nursery becomes a dojo, which becomes a social enterprise cafe, which eventually becomes an estate agent, which then closes.

Rents are high, parking difficult, the products undifferentiated (carbon copy list of the same 15-20 chain franchises), not unique. After all of that, I'll assume prices are no better and may be worse than online retail.

I'm not sure what the answer is, as retail is not my area of expertise. However I think that if there is a solution it will have something to do with people, community, and offering a specifically local experience. Not one which is purely a chore, and I think it will involve much more than just "convenience".