how much would it be to add dads car onto your policy for the period of time you need it? i'm just thinking, surely its a fairly common thing to do if you want to give a car a thorough test drive before buying it, you might need to insure yourself on it for a few days first.
might be cheaper than you think, given your own insurer will know you can't be driving your own car while you're driving another car.
or even phone up to swap your usual car for your dads car, in which case you might even get a refund if dads car is less risky than your current car. then swap back when you're done. though, this would, i guess, leave your current car uninsured for a few days, so might not be preferable unless you can lock it away in a garage or something.
but long story short, i'd definitely phone your insurer and see if they can help.
another thought - take out a separate policy to start the day before the holiday, then cancel it when you're done. you'll still be in the cooling off period, so might only have to pay an admin charge of £30 or so.