And this is why the Republic of Ireland and Northern Ireland are considered 'home turf' for the operators of the UK and the Republic in a nice cross-border arrangement. Kent occasionally has a moment when the bits closest to France sometimes find themselves with a roaming text message 'Welcome to France!' despite sitting in their conservatory or their front room.
And of course, this is where the EC has done the right thing to enforce a single cross-border tariff agreement for mobile roaming, which we are regrettably no longer a part of. There are some moments where a certain Spanish operator does not consider Switzerland to be part of the EU (rightly so, they're not) and thus excludes the country from its EU roaming plan, causing much agony to those who flew into Basel and who oscillated between French (yay), German (yay) and Swiss (boo!) cell towers...