Sod getting a roll cage unless you plan taking it on a track/strip. Unless you are some kind of poser As above though it will screw your insurance too.

But more important is I would advise you to get your own insurance sooner rather than later. Insuring yourself on your mums car as an additional driver is not a smart move. It might be cheaper. But getting your NCB started is far better for the future.

Most people have an accident in their first car. So I dont think your mum will be too pleased either. So save the £300 in will cost for a decent cage and put it on your own insurance.