Japan has a $5 trillion economy, while the UK is $3 trillion.
Yes. On the other hand Londons significance as a financial center, the Commonwealth and British soft power should count for something. British media and cultural products are a lot more influential than they ought to be based on its economy and population. British universities attract a lot of foreign students and top researchers. British "systems" and ways of doing things are still the model for a lot of former colonies etc. All this is declining, but still a factor.