To explain/sanity check why I think it doesn’t make sense for me personally to do this:
A thing the UK has is ISAs, which are accounts that you can put a certain amount into per year and not pay tax on your gains (or interest, for fixed-interest ISAs). The limit is something like £15k. One of my ISAs lets me invest in various stock indices, but not individual stocks, and I’d be surprised if there was one that let you pull off a trick like this.
Separately, any money I put into my pension lets me reclaim the income tax I already paid on it. Then I think I need to pay tax when withdrawing it, but some of it is tax free and the rest will be at a lower rate unless I withdraw a lot or have other income. (Or I guess if the rules change, which is maybe a possibility I should take more seriously.) My pension also lets me invest in stock indices.
So as I understand it, the bulk of what I invest should (and currently does) go to an ISA or pension. It’s probably not harmful to put moderate amounts in the kind of broker that would let me do this trick, because I only pay capital gains tax if my gains are above some threshold. But I suspect the intersection of “enough money it’s worth doing this trick with” and “not so much that I should just have it in an ISA” is either empty, or narrow enough to be only barely worthwhile.
(I’d also need to figure out where to put the money I’ve borrowed. We have current accounts that pay a few % on up to a few £k, but for large amounts the best safe investment I can think of is maybe premium bonds. That’s a lottery that pays 1.4% interest on average, tax free. You can invest up to £50k in them. But then doing it with a smaller amount is riskier, because there’s more chance that I don’t earn enough.)