You've hit the two main ones on the head but the third is more intangible. In London it seems (to outsiders) that people are just trying to emulate SV, this means that you're competing in an international and, most likely, saturated market that doesn't align with what the rest of the UK is traditionally best at.
The UK was built on trade and engineering that mostly started outside London to avoid the Guilds and their stupid rules. I think being outside London gets you closer to these skills and experience and makes you focus more on building a business and less on following international, sexy hype.
I say it a lot but the best place to make money is in unsexy markets, and outside London there are a shed load of unsexy markets desperate for young start ups to dive in.
Don't get me wrong, there are some incredible companies in London doing amazingly well but there is a lot of dross too.
