Signage exists. Plus the vast majority of times painted ones are used in areas where almost everyone on the road knows how it works, and within minutes of it snowing a very clear outline of the path cars have taken would make it abundantly clear what the process is.
Roundabouts are engrained into UK road culture, you'd seldom find a driver in the UK that cant figure out how one works, even if they may not have great lane discipline on the larger ones.
People are generally driving significantly slower in snow though, so the need for a roundabout is lessened. And you can also install signage indicating a roundabout is there.
A roundabout requires signage in any case. At least in all countries I've seen one so far. Otherwise it's not a roundabout and may even have very different rules.
The signs preceding a UK mini roundabout would not be.
And it rarely snows in the UK these days. And I would hope you would be driving extremely cautiously if there were snow on the ground (in the UK) as it's such a rare event.
I'm not sure what your point is as roads become invisible when it snows. Is there something unique about a mini roundabout versus any other road markings? It's almost as though you're implying that drivers will speed towards a multi-road junction when it's snowing and not bother to slow down, despite the signage.
