That is frustrating! It is annoying that your HCP didn't check first on your phone compatibility before issuing the Dexcom One. I had a quick look at the Dexcom website and only Galaxy devices from Samsung are listed as supported as of now. If your phone is a very recent model, it is probably still in testing by Dexcom. I've been a Dexcom user for a few years and have noticed it can take up to a year for them to complete testing and give compatibility OK to either new phone models or to latest updates in OS.
Please check with your diabetes team whether a Dexcom reader is available for the Dexcom One (or contact Dexcom Uk directly). I had to resort to one for my G6 a couple of years ago, when Apple radically changed the hardware in the then latest iPhones. It was about a year before I could go back and reinstall the App on my iPhone.
Alternatively contact Abbott to see if they can support your phone - if yes, then ask to be switched to Libre 2 (I believe this also can provide a reader if needed under the NHS).