Kalin,
You haven't said if you have a disability? If you do, you can get financial support for your driving lessons and even towards a specially adpated vehicle to provide for your needs. Also if you have a very good instructor, they can also advise on getting in and out of the vehicle easily and help with operated adapted controls. If you are driving your mothers motobility car then I'm assuming you have provisional insurance cover for this. If this is so, the likelihood of you getting full insurance cover is promising as you can always call up your provisional licence insurer and ask if they would take you on after passing your test. If you have a disability then you will be given some extra time for your practical test and the examiner is also trained to understand special needs that arise from disabilities.
As far as driving a motability car without having any disabilities, this is legal as long as you are a nominated driver. Customers of motability cars are allowed two nominated drivers and assuming your mum is the first, you can be the second. A third nominated driver can also be added for an extra fee but if a nominated driver is under the age of 25, there are restrictions in the type of vehicle they are allowed to be insured on. I hope this is of some help to you.
