This is quite a common occurrence in border areas, where a foreign network provides coverage as well as your home network. The "welcome to your host country" text message as much serves as a warning in these scenarios as it does a cost notification when you are abroad.
The following two past threads discuss this in more detail, you may wish to read them - https://community.ee.co.uk/t5/Mobile-Network-discussions/Isle-of-man-roaming/m-p/1389233#M34512 & https://community.ee.co.uk/t5/Mobile-Network-discussions/Roaming/m-p/1237051#M26246
If you wish to completely eradicate this "domestic roaming", then manually selecting EE's UK network will resolve - at the expense that if & when you lose EE-UK coverage for an extended time, your phone will not automatically reselect and will remain out of coverage until you manually interject.
If you leave your phone on automatic selection, it will regain EE UK coverage automatically within a few minutes of it becoming available. The downside is that if you ever lose UK network service when a foreign network is available, your phone will roam onto it - by design.
The choice of which scenario is preferable, is yours.
