Any BMC (Part of my post did not appear - this is a list of marques ruled out.)
Ferguson
Singer
Hillman
Humber
Rover
Alvis
Jaguar
Vauxhall
Triumph
Daimler
Lanchester
Lagonda (?) (I'm not sure if this has been ruled out)
There's not much left - I'm personally ruling out Frisky. Those wheels look like they're from the mass-produced range of the maker. Standard looks like a good call, they were always considered a little bit upmarket from Ford and BMC.