I'm a Yank watching the UK version of Being Human and the character mentions sitting next to his ex-girlfriend in "Double History" (season 2 episode 3 around timestamp 24:18). It's clearly a history course of some sort, but what sort? I've never heard the phrase before, and duckduck and google are both failing me.
This refers to having a "double period" of history, or two periods in a row.
When I went to high school in Australia in the '80s a period was typically 40 mins, with 8 per day. Double, and occasionally triple, periods were scheduled to allow for a longer uninterrupted block of teaching time.
The reason for using the phrase "double history" in the context you describe may be nothing to do with history per se. It's more likely used to suggest to the audience that the student in question considers a double period to be an excruciating length of time to have to sit next to your ex-girlfriend.