d1sk1d, I'd accept that you can not drive your mother's car, whether she's in it or not, unless:
She has you as a named driver on her insurance
OR
You have a policy on your own car that gives you the right to drive any other vehicle with the owner's permission. (Be very careful with this - it used to be the case that any FC policy gave this cover - that's not the case any more, you have to check the small print). In any event, as Eddie has said, that's only 3rd party cover.
I can't think of any other way in which you'd be legal. Ignore what you've been told about learner drivers. Again, as Eddie says, that is total hogwash where insurance is concerned.