Changing countries with Phone?

So, I'm assuming this will be fine but I'll ask considering it's someone I know's phone. So she has a BB and is moving from Canada to England for a few months. Since she is gonna put her contract on hold, if she brings the BB to England, unlocks it and gets a new sim card. Could she get a plan for over there, and when she comes back put in her old sim card and just take it off of hold and it will be fine?