Most finance companies set interest based on risk. Higher risk, higher rate. Based on this and your low credit rating, 13.3% isn't exactly extortion.
That said, I don't necessarily agree that you should proceed. As Mr Big said, it depends on how much you want and need the car.
Do you really need this car? You can buy very reliable cars with MOT and road tax for a couple of hundred pounds.
You gotta tie yourself to the mast my friend, and the storm will end.