There's a handful of past threads on this board with the same query, north & West Wales is another regular spot.
This behaviour can often happen in border locations where there is no coverage from your home network, but a foreign network serves.
For as long as your home network has service, this should not happen. If you lose coverage, your phone will naturally attempt to roam by design. The "welcome to country xyz" texts are partly intended to provide notification this is happening.
When your phone regains EE coverage, it will stop roaming. A manual network selection performed when in EE coverage prevents unintentional roaming, but when searched outside EE coverage should serve to confirm lack of service.
