Adam - congratulations on passing your driving test!
My daughter is 18 and just about to take her test but we put her on her father's car insurance with Tesco as all the time she is on their she is building up her no claims bonus (I think Direct Line do it as well). I think if your Mother insures the car as her second vehicle that her no claims are not taken into account but it maybe cheaper that way, also you could build up your no claims as well.
Sorry don't know about the legalities, let me know how you get on as I am going to do the same for my daugher.
PS Shiverkitten .. that's a bit harsh as we don't know his circumstances. My daughter is a brittle asthmatic and sometimes cannot walk 20m but I can take her to college in the car and she can still attend.