Re: Just popping over to Calais ...
The operators have thought of that. On the SIM-only deals I've looked at there's always some small print to the effect of "if you mainly use this abroad we reserve the right to cancel it:".
They can cancel any contract for any reason as long as they respect duration and return any positive balance. They generally make money when you roam so there's little incentive for them to stop. I've had an Orange PAYG for nearly ten years here which I use for SMS with the folks. Never had any hint of a cancellation and it only gets around £10 a year.