Interesting. I consider that an actual safety issue, as opposed to safety issues fabricated solely to give cabin crew more control but safety isn't actually implicated.

What do UK airlines do if someone won't return to their seat when the seatbelt sign comes on in flight? Physically wrestle them to their seat and buckle it for them?

I think that if they know someone is in the bathroom and hasn't come out after a minute or two after the seat belt sign has been put on, they will typically start knocking on the door. Conversely, if you are queueing for the bathroom and the sign is switched on, they will routinely tell you you have to return to your seat and can come back when the sign is switched off. Even more obviously, if the sign is on and you try to get off your seat to go to the bathroom, they will typically tell you you can't (point noted about the exception reported above for an elderly lady but note that this is definitely not the norm and the staff could have got in trouble for it).

On AA (and other US airlines apart from take off and landing specifically people are routinely allowed to go to the bathroom is the seatbelt sign is switched on though sometimes pointing out to the sign and saying it's at your own risk. I'm sure there are exceptions too (e.g really rough turbulence) but it is the norm.

To be clear, I'm not saying one or the other is better but simply that the airlines have different priorities and even more obviously obey different regulations by different authorities. Also noted about the "toilet in your own cabin" restriction being removed! I'm sure I may come to miss it!!!

I was thinking it was due to there being ice in the OPs drink.