Which was first?
I saw in a classic magazine the historical statement that the first British road car with four valves per cylinder was the Bentley. While accepting that all vintage Bentleys had such engines and respecting their competition achievements, so notably at Le Mans etc, perhaps the assessment should be taken with some caution.
The first Bentley 3-litre engine was designed in 1919 and started up on the test bench that October. A test car was ready for The Autocar to try by 1920, but it was not until late in 1921 that a car was delivered to a customer.
Louis Coatalen had decided to ginger up the 24/60hp Sunbeam. Since the war it had been such a well-liked road car. Its side-valve engine was changed to incorporate push-rod operated overhead valves. A tuned version was also offered. In 1920/21 he offered also a single-overhead camshaft engine with four valves per cylinder, Type OV, for the 16hp 3-litre and 24hp 4 ½-litre Sunbeam models. Like WO, Coatalen realised that multiple overhead valves were the way to go for racing cars, and maybe he wanted to see how customers would respond, which was probably why they were offered the OV power unit in a 24hp Sunbeam at no extra cost. I admit I do not know which car, Bentley or Sunbeam, was sold first, but it does seem to be a factor to consider.
While Bentleys dominated at Le Mans, etc, Sunbeam was successful, for a shorter period, in grand prix racing, notably as the first British car to win the French classic, in 1923.
While on this subject, there were a few 16-valve Bugattis made by Crossley in Manchester – I have a Crossley-Bugatti radiator badge to prove it. So another British contender?