As i understand it, the situation in the US is that there is Silicon Valley, where the salaries are high and the rents higher, and then half a dozen other places where there are significant concentrations of software jobs, and more reasonable living costs.

This is not really the case in the UK. There's London, there's a ring of big corporate offices just outside London (Guildford, Cheshunt, places like that), and that's more or less it. You pretty much need to live in London or its commuter belt to work non-remotely in software.