I have been wondering about the UK only feature when it comes to writing jobs. When I post jobs requiring English fluency, I usually post two jobs - UK only and worldwide. I'm concerned whether UK or US only features are being used to find native speakers specifically for this type of work, when there are plenty of native speakers abroad that might miss out on opportunities for work. I know you can set up jobs that specify the level of English ability but is it as commonly used? Are there any statistics for this? I haven't really looked into it but it's been on my mind.
UK only means the client wants only people physically located in the UK. Nobody can tell you why a client decides to do this, but people who have been around the block on Upwork a few times can make some educated guesses.
It's not just about the level of English but also the type of English. UK only assumes British-style English which tends to be the choice for Australians, South Africans and others as well as Brits.
I'm not clear on your question exactly, but as a native English speaker who moved to France, I'll contribute two cents! The designation for UK only is really for location, and you'll have freelancers not physically in the UK avoiding those or not getting invites. You probably don't want to do that if you're open to native UK English speakers that might live anywhere. So as a freelancer I'm going to pay attention to the description for whether the job requires UK, US, or some other English dialect. Just select native level English, then specify in the job posting which you're looking for.