Depending upon what model phone you have you may be able to accomplish your goal fairly simply. I am assuming you have a dual sim phone however.

If so, and you have WiFi calling on your UK voice line, you can keep it active but assigned to a non roaming carrier partner in the US. For iPhones, this is under: Cellular-UK Line-Network Selection-Automatic to Off, then chose a carrier you cannot roam on with your UK carrier. Android has something similar. Once your UK line cannot connect, it should default to "Using Cellular Service" of the roaming data sim. At this point, the data sim is behaving like a WiFi network for your UK WiFi calling. This also assumes you have free or discounted WiFi calling on your daily voice line.

I have US service and this system has been available for several years on iPhones and some Android models. It's called IMS for Apple products and something like Standby Calling for Android. Once set up, it's rock solid for function and awesome for travel. You can test it prior to departing for configuration and function with any two available lines. There are several discussions on Flyertalk over the past few years if you care to investigate further.