iPad Contacts app incorrectly formats telephone numbers
The phone number format is set in Settings -> General -> International -> Region Format.

But believe it or not, it seems like your carrier is actually able to override whatever you've got set there (on my WiFi iPad, changing the Region Format updates Address Book's format, but on my iPhone it doesn't have any effect).

While researching it I saw some mention of Verizon fixing this issue by dialling *228, waiting for it to connect, then dialling 1 and waiting for it to disconnect.

Is it a 3G iPad? Perhaps while you were in England, something got switched over.