[rootusrootus]

That's not specific to eSIMs, that is an iMessage issue. Having done this recently myself (just got home yesterday from a trip to the UK), my recommendation for anyone about to travel with an iPhone is to get a data-only eSIM that works in your destination country, then toggle off roaming on your primary line and make sure WiFi calling is enabled. If the phone asks to add a new number to your iMessage, refuse. Then your phone just works like normal, including iMessage, but without the roaming charges.

Oh, and if my experience is typical, don't go with O2 in the UK. I was disappointed in both the coverage and the performance. Even in London proper it was frequently crappy.

[preordained]

That sounds terrible! Some people just take these things in stride I guess, but I have no interest in going through these kind of hurdles for something like this...