As someone with experience of this, it's easilly doable.
The real problem comes from learning the course in the wrong order. I did Maths, Chemistry and Biology in a year. I was doing C3 while doing C1. F321 before F324. It definitely made things a lot more complicated.
If you have the option, definitely try and self teach it in the right order, at least in one subject. I don't really know how it is with humanities subjects as the closest I got was English Language A Level, where you could easilly do module 4 without doing module 1, but the Sciences are very dependent on the module before it. If Module X depends on knowledge from Module W, make sure you know the bare minimum from W to give you a foundation at least, no matter how rickety.