Pre war Le Mans style Bentleys

[Dr Jekyll]

I saw a couple of these going down the M40 this morning, several miles apart. It struck me that while every other example of a pre 1960 car I've seen on a motorway has been trundling along the inside lane at 40 or 50, every time I've seen one of those Bentleys it's been bombing down the outside lane overtaking everything.
Is there a generic name for open top pre war Bentleys that look like they belong at Le Mans, or do you have to know what size of engine before you know what to call it?
How many are roadworthy in the UK?
What do they do to the gallon?

[Dr Jekyll]

That basic design dates back to the days of the blanket 20 limit though.