Can I ask to start my postdoc remotely during an interview? (Postdoc is in the UK and I live in another country)
I wouldn't be concerned. This is a common concern these days, so I'm sure they will understand.  It might be that they tell you that it is not possible for legal reasons.  It might however even be that they are happy to know you are willing/interested to work remotely for a while, since under the current circumstances, there might be significant difficulties/delays in getting you into the country in a timely fashion (depending also on your passport, and where you travel from).
Regarding as to how to bring it up, you should do so as a question - like &quot;Would it be possible to work for a month or two remotely, given that it will be difficult to relocate (and other reasons)&quot;, but it is certainly good to present it in a way which will make it also look favorable to them, i.e. &quot;give us more time for the relevant paperwork&quot;.
Of course, this all also depends when the position is actually meant to start. I assume it is soon, so chances are high that restrictions are still in place at that point.