It is free to people in the UK (you don't need a licence, so it doesn't bring in any money) so why block others?
Isn’t it that people in the U.K. fund it through the licence fee? So it is a service provide free to us as part of the range of BBC services (like all of radio).
But the BBC can conceivably sell radio programmes to broadcasters in other countries or as audiobooks etc. That’s a potential income stream, but not if all the world can listen for free on Sounds.
There are lots of YouTube channels and news outlets internationally I can’t watch “because content isn’t available in your area”. I think it’s not really unheard of to restrict licensing to particular areas/markets.