What can go wrong?
Well lets take this for one.
There is a well known village in Kent that has frankly shitty UK Mobile reception. On the otherhand the french networks are loud and clear. So you have a prang and the car connects to Orange FR and sends it details off.
The french emergency people fire up their Pompiers et Sapiers vehicles only to find their way blocked by 1) striking dockers at Calais
2) La Manche.
Repeat for any prang close to a border with another country.