James,
Pip pip and jolly good show for understanding the poms.
Yes, the chaps do call the "hood" a "soft top" something to do with the fact that it is made from material and is soft as well as sitting on top of the car.
As far as the "bonnet " is concerned, who knows, probably some tradition originating from the Magna Carter.
Quaint and all as it is, most of us Aussies call the metal flap that covers the engine a "bonnet" and the folding thing that covers the driving compartment and keeps the rain off your head - the "soft top".
Anyway, I`m off to the cricket- another English invention where the batting side is in, with the fielding side, who just got out, trying to get each batsman out so they can get in. And you reckon hood and bonnets are a problem.....
Cheers