From what I have gathered,even if the car does not belong to
you,you still must display your probation plate clearly if you are
a new driver.Using "driving a car that doesn't belong to me thats
why I did not display a probation plate" as is reason will NOT be
accepted and the law will be used against you accordingly.Thats why
I have friends who carry their P plate around with them wherever
they go cause they're constantly borrowing cars. 
I stand corrected if I am wrong.