Here is another car I'm not familiar with. It sort of looks like a AC Greyhound prototype, but what is it?
I think you're probably spot-on there.
Aft of the A pillar it's almost completely AC Greyhound, whilst the front is AC Ace.
The registration number 3 FGP is a Surrey issue (AC were in Thames Ditton, Surrey) dating from April 1958; since the Greyhound came out in 1959 the timing would figure as well.
There's something very disjointed and half-built about that A-pillar and windscreen arrangement. Definitely not ready for production. Why the steel wheels? Also it's not Bristol-powered because there's no intake for the tall engine.
If you read a certain site about AC history it mentions the Flat-4 car I posted about and I still think this could be it, lashed together by the AC works using bits of an Ace, and bits of a Greyhound. Oddly enough, Georgano doesn't even mention the proposed flat-4 and flat-6 engines.
PS That flat windscreen doesn't seem to come from any AC model of the 1950s. I suppose you all know that's Goodwood.