I have worked in insurance for the last 5 years- as previously pointed out it is called fronting and also classed as insurance fraud. Although the chances of them finding out are very slim, if they do they can refuse your claim, void your cover and cancel the policy. You then need to declare to other insurance companies why you have had insurance cancelled by another company, which affects your ability to get insurance later in life (not just motor-all insurances).
I do not know one insurance company who would be fine with is- the reason I say this is that you present a higher risk to them that your mum. The premium you pay is based in the risk presented. So essentially if you mum is the policy holder, you are paying a much lower premium compared to the risk that they are taking on (obviously you being a named driver is factored into the cost).
I am sure if you asked Admiral if they are happy to take on a fronted policy and recieve less money from your for the same risk they would say no.
As soon as they suspect fronting (car has just been purchased, young driver on the policy, more than one car at the home etc) they automaticly charge you a bit more anyway.
Although the chances of you getting found out are slim do you really want to risk it?