It is ILLEGAL end of, as is FRONTING.
The reasons why the conversation is recorded when you take out insurance is certain questions are asked such as MAIN USER, any lie can void the policy, but while insurer is getting paid they will take, then just say NO when a claim comes.
It is a myth that fully comp covers you in any other car third party, some do some don't, I have fully comp any car any age insurance group etc.
Most fully comps do but not all, needs checking with individual insurer.
Fronting and the above practices are being attended to, one of the reasons why askmid.com was made available to the public so they can report non insured.
It is true however that the plod will have hard job as when they ring the insurer and they confirm insurance is in force the police are NOW allowed to state(eg) does it covers alloys bodykit stereo gear etc etc , then insurer can decline on the spot as details of mods not passed on, and they are nicked.
tabetha