Rumour has it, I have a 10GB download limit. But my ISP never seems to penalise me...?

The 10GB limit is simply a fair use policy, it doesnt mean you can't exceed it, and it also doesnt mean you will be stopped from downloading anything, this is just an average of how much you should use it "fairly" for what you're paying. If you go over this, your provider may contact you and ask you to reduce your downloads, but legally there is nothing that can be done to stop you going over it.