Yes that's RAC Horsepower. It's not measured horsepower, it's calculated from the formula RAC HP = 2/5 x D squared x n, where D is cylinder bore in inches and n is number of cylinders.
Cars like the Austin 7 were named after their RAC horsepower rating.
Because the taxation figure was dependent on the bore (and not the stroke), it led to a generation of British engines with very small bore and very long stroke - the opposite of what the Americans (and VW of course) were doing.
The British government abandoned this system in the late 1940s.
https://en.wikipedia.org/wiki/Tax_horsepower