Go back a few years and remembe a thing called "the great depression" and look at how many motor vehicle companies failed to survive.
During that time BSA realised that the days of high profits were gone as were the days of specialist motorcycles .
They also realised that they were marketing up to 20 models with little other than the gearbox hub retainer common to the range so they set off on the rationalization path.
The OHC was one of the designs that made it into metal for evaluation .
The idea was to use as many common parts as possible across the range.
Thus the commuter C range in 250 & 350 , the sports B range from 250 to 500 & the M heavyweight range from 350 to 600 .
Pre war sales showed that the 350 C 12 was too expensive for the commuters , the 350 M 19 too underpowered for the chair haulers & the 250 B 20 21 & 22 not competative so all of them got dropped from the post WWII line up
The M24 Gold star was untouchable pre WWII so became the B 34 post war.
There was no way the board would authorise making an OHC 500 to challenge their own very successful push rod 500 as a stand alone engine at a substantially higher price and a 350 version would have been uneconomic .
Tooling for a new model was (and still is ) very expensive and the predicted market sales would not have covered the tooling costs let alone developement cost plus make a profit.
Remember in the UK it was "production racing" that generated the sales not exocit one off factory race bikes .
The same logic was applied to the decision to recommence production of the already obsolete C range rather than the cut down A 7 and in that case the A 2.5 & 3.125 would have been quicker & cheaper to make than the C range but the board could not wear tossing the C tooling out at a loss .
They were making C's as fast as they could and were selling them all every season plus the tooling was not due for replacement & had not covered it's developement costs .
Add to that while BSA was flush with cash, the UK was bankrupt
As such the UK government did two things
1) introduced a massive tax on undistributed profits , designed to get more money flowing in the internal economy
2) retained the ministry of supply dsigned to restrict materials , much of which was imported so it could have been diverted into export goods marketed into the USA as the UK had a massive debt to the USA that had to be paid off
So every pound of iron, steel, copper & aluminium had to be authorised by the goverment .
And from memory the ministry of supply remained there till the Thatcher government.