TheTallOne, has pretty much said it all really.
I have 27 hours a week this term, whereas last term was about 20, just depends which modules you choose. It is quite a lot more than most degrees, things like history have about 10/12 but then again we have no extra reading to do....
I would definitely not recommend living at home. I have some friends who have done that and they feel like they're really missing out on the uni life, also we have 9AM starts pretty much everyday, and buses will be really busy/slow at that time of morning, so you'd have to leave really early. I suppose it is possible but from what I've heard I wouldn't recommend it, unless your willing to miss out on alot of the uni life.
Maths degrees generally are fairly hard, mainly because they're very different to a-levels. But it's not overly hard, not yet anyway :P especially if you go to all lectures, supervisions etc
You'll be pretty lucky to get in with a 3. I agree you should definitely do the AEA, if you've prepared for step, it makes AEA much easier, and you don't have to put in much more work imo.