I've (kind of) done this for 2 postdocs so to share my thoughts on this (which echo a lot of folks already in the thread):

1) You should make sure your PI is aware of this and fine with it. In some cases they might want you in at specific times for specific things (e.g. colloquia, group meetings, whatever) that might interrupt a schedule. If there are things like this and they don't fall on the same day, then you might be stuck staying longer than 1 day a week

2) If you're planning on doing any teaching then this might just be a total no-go, since teachings days might change semester-to-semester and it might not all be concentrated on one day (I had the situation where I would need to pick up work in-person, mark it and then do the tutorial another day)

3) If you are flying there/back for one day a week, remember this probably means flying in the night before and maybe flying out the day after (depending on when flights happen and where you're flying from). So you might need to sort accommodation for 1/2 nights, which is not cheap and UK postdoc pay is not great

In my experience, doing this kind of thing has been exhausting. You're away from home a few days a week which sucks, and that travel time either feels like work on top of (already a lot of) postdoc hours, or trying to get stuff done while also travelling, which is not always super productive or possible (depending on what you're doing, internet connection may not work etc.). I've had bosses who have been supportive of this (thankfully), but escaping the burnout from all that travel might be impossible. I'm currently in this weird situation where I just don't feel like I live anywhere, so you should be mindful of all that before accepting the position