whose who
It cannot be that an insurance company sets the law, only the CAA in th UK can set the rules. Insurance companies do not in my experience attempt to do that. Of course, the law requires that insurance is in place.
It is reasonable for an insurance company to protect itself and so may require for certain types, that have proved the need, a demand that training is first given on the type usually for a set number of hours before they will provide full cover. It's my experience when this requirement is put in place that the training must be with an instructor. How otherwise can the Insurance Co be sure that the training is to the mark unless a person who is qualified to give it gives the instruction.
We also need to consider the law: ANO Schedule 8
Private Pilots Licence
(i) he may fly such an aeroplane for the purpose of aerial work which consists
of:
(aa) the giving of instruction in flying, if his licence includes a flying
instructor’s rating, class rating instructor rating, flight instructor rating
or an assistant flying instructor’s rating; or
Now is the fellow group owner or another with experience on type allowed to give 'instruction', if they are not an FI or CRI? When an insurance company requires instruction to take place then that is what they mean - instruction. I have never seen the word 'checkout' used by an insurance company but I'm here to learn. Unless with certain types such as the turbocharged Lance and others the insurance will only require the pilot to comply with the law.
Is a checkout 'instruction in flying'? The purpose of insurance is to protect oneself and others. I wouldn't be too keen to risk the argument over symantics.