Presumably, if you want to "build hours" you want to act as P1.
The insurance company are entitled to specify any experience requirements that they like for the P1. 250 hours sounds a bit high for a simple S/E 2 seater, but that is there choice. You cannot fly without valid insurance (legally).
Why do you want to build hours? It sounds like your "deal" is to fly only whilst the owner is also onboard. Is this really P1? The full ambit of P1 is to have total control. Unlikely, if you always have the owner onboard (who is himself a licenced pilot).
There is a reason why proper hours building is a good idea. you learn from that experience.