HI Lucy,
It's dependant on what your filling your barrels with or if they are already filled'
Provided your medium in them isn't saturated or frozen hard I think you will be ok to plant them up now. Strawbs are hardy and your apple should be dormant so you should be ok ...Make sure to tease roots open on apple tree and tamp round it so you dont leave no air spaces around roots.. If your tree has already got shoots on, You could fleece it if frost is forcast.