No one city/area in the rest of Europe is a serious competitor to London on both salary and diverity of roles available, not to mention tech investment and funding. That's no guarantee of that continuing of course, but the difference is stark at the moment.

"Both salary and diverity of roles" is your criteria. Another criteria would be for example that the average tech salary in Zurich is double as high as in London.

I don't think this is true. You can find well-paying jobs at very well-funded (or profiable) companies in other tech/science/finance hubs around the UK (Cambridge, Oxford, Bristol, Manchester, etc.).

I think you misunderstood, no one ever claimed that every region and every company in the UK outside of London was worse than every region and every company in north western europe. It is more a statistical statement.