They will sign off on the spot as Norman says. I am not sure what
extra policy you are contemplating I do take extra coverage with me
through my travel insurance. I did have an accident in Tasmania, but
that time my coverage had been arranged locally so I do not know if
supplemental insurance was a part, but I did not have to pay even
though the front corner of the driver's side was clipped by a right
turning vehicle.
Thanks (to you and Norman).

An example of the insurance I have in mind is
www.questor-insurance.co.uk It seems to be pretty much a UK-based
product - possibly driven by the number of Brits who fly/hire rather
than enduring a channel crossing.

--
R