Well we still used RAC h.p. in the 1950s when referring to engine size although vehicle taxation no longer depended on it. e.g. Lea-Francis 14 and 18 h.p. saloons, Alvis TA14 and TC21, Morris 10 etc.
The Gough engine was a 1½ litre and (I think) 69mm × 100mm so 11.9 RAC hp but 78 b.h.p. The 4.3 litre V12 Lincoln Zephyr engine used by Atalanta (and Brough Superior and Allard) would have been more than 30 RAC h.p. I think, but I can't find its dimensions.