Your iPhone 11 is a dual sim device. Install one line as an esim, the other in the sim slot. Assign your Mint line to a non roaming carrier while in the UK and your UK line to be your data line. For the Mint line, do this in "Settings-Cellular-Network Selection-Automatic OFF" Then pick a carrier that has no roaming service. This will allow your Mint line to have a pseudo WiFi connection through the UK data service. As long as Mint has free WiFi calling to and from the US, it should receive calls & texts from the US with no charge. You only need the iPhone 11, not both phones for your goal.