Must be a Vauxhall. A prototype?
Answering my own question: Vauxhall Vettura prototype ca 1965
That's right.
The future face of Vauxhall is there to see way back in 1965, 8 years before the original droop-snoot Firenza first appeared.
Here's a picture of it with its 1965 registration, which is actually only a handful of digits away from the publicity photos of the new Vauxhall Cresta PC just announced and on which this one-off prototype was based; the car was broken up without ever being seen publicly:
