One thought occurs to me: what happens when your phone switches from the local (i.e. your home country) network to an overseas one (with corresponding hike in tarrifs)? Is it a sudden switch-over or is there a dead zone? Could this happen in mid-call? I presume for UK residents there's always going to be a dead zone over the Channel anyway.
Last week we came across from France on the ferry, so the question of prohibitions didn't arise. But, knowing that I'd lose cover as soon as we were out at sea, and not needing to make any calls, I dutifully switched off as we set sail and didn't switch on again till we were docked in Newhaven.