A cross-flow head will be more complicated to design, and probably to cast, especially for a push-rod engine. And if you really want the best advantage from it, you need the valves not to be in a single line - the TR7 engine has awkward exhaust porting because of the single cam. That makes a "proper" cross-flow, like the Sprint (or the Rover 2600 that was derived from it) quite a lot more expensive to produce. Post-war, when the Triumph SC engine was designed, it was worth it for Jaguar but not really for anyone else.
(I don't know about Bentley but Skoda built engines with four valves per cylinder way back in the 1920s)