We tried Adrian Flux when we first got my Saturn to the UK. They would NOT insure the car because they didn't know what it was, even if pointed in the right direction. 
Eh well 
That's odd.
Adrian Flux are usually pretty good on things like that, and at least know what you're talking about (unlike one broker, I forget which, who kept asking me "Pontiac? What make is that?"
).
Post office? No kidding! Do they allow the occasional use of other insured cars as well, do you know?
The "other cars" extension has always been pretty much a standard clause. I understand that some insurers are now not including it in policies for younger drivers though.
You'll hear a lot of misinformation about this aspect of coverage too. Some people will say it's only available on a comprehensive policy, not TPF&T (third party, fire & theft), which is not true at all. Every U.K. TPF&T policy I've ever had since the age of 17 has included it.
A lot of people also seem to have the idea that you are covered to drive somebody else's car only if that person has his own policy for the vehicle, which again is not true. Unless your "other vehicles" clause specifically requires that (and I've never seen one which does), then you're covered regardless of whether the owner has insurance or not.
