You have 5 AS levels, and so you definately have room to drop one if necessary. And seeing as you have History, English Lit, although a great subject respectability-wise, is not really necessary. You have Maths, Further Maths and Physics which is really all you need, and History for some variety. Seeing as you want to do maths, those A-Levels won't look bad at all.
I say that if you do a module in January, do that and see how you do, and then carry on for the rest of the year if it's good, and you want to, but if it's brought down your other grades, of if it's a bad grade in itself, drop it then if possible. if you don't do January modules, then I see carry on till Christmas and see how you feel then-it might pick up. If you're really not happy by Christmas, then drop it then-talk to the examination officer about it.