In Being Human US S01E12, when Bishop attacks Aidan, Bishop gets hurt by sunlight and starts burning. But in the whole 1st season it never occurred, vampires walk around freely in sunlight, so why did it suddenly happen?
It wasn't sunlight. Bishop burned because he crashed through the window and entered the house without invitation. In the Being Human universe:
Houses owned by humans or werewolves cannot be entered by vampires without the owner's permission, unless the vampire itself either lives there or was turned there. Entering uninvited for more than a few seconds will cause the vampire to burn, and can fatally melt the vampire away if they don't get back out quickly.
Bishop didn't die because he escaped right after attacking Aidan. Also, I think the scene takes place in the evening anyway (the exterior shot looks dark and the porch lights are on) and Bishop's face only starts to burn when he's inside the house.