If your insurer doesn't want to know then you may have to change either the insurer or the level of cover you have. For example, they may agree that you can change to 'any driver' rather than a number of named drivers. This will cost, though.
Here in the US auto insurance generally covers the driver, not the car. Thus it's possible that if your friends have US auto insurance it will cover them in the UK. They should check with their insurer, too.