Dashie

Yep, completely at the discretion of the insurer, they all have their own underwriting criteria. The company I used to work for wouldn't add an overseas licence holder as a temporary driver. However, we could add them as permanent drivers, and then take them off again when the need was over, so we got around it that way. The loading for overseas was hefty, and resulted in a big premium, but most would be refunded when the policy was changed back to normal, so that could be worth a try.

You can't take out your own policy to cover yourself because it's not your car. By law there needs to be an insurable interest, ie ownership or liability, in the thing you insure. If you were to crash the car, it's not technically your problem, so they can't pay you, they can only pay the person that the car belongs to, as they are the person who has lost out. If that makes sense.

If they do agree to cover you, request a cover note or certificate showing that, they should issue that automatically anyway.