Testing at 250v is only beneficial when there is a fault so that you don’t need to subject circuit to 500v however you need to carry out insulation resistance test at 500v to record accurate fault reading and true scale of insulation. I’m trying to test you here ?On both tests at 250v and 500v. I tested live to earth at 250volt seen that it was very low near enough zero, now this has been connected to a RCD and MCB for years this circuit. So I then joined live and neutrals together at consumer unit then tested live to earth IR test at nearest socket comes up >999mega ohms. So I am thinking I’m testing through the USB points when carrying out live to earth only with neutrals still connected to N bar ?? That make sense