the B series engines are half side valve, as I mentioned there is a very specific reason but damned if I can remember...the two things that spring to mind, and they are guesses on my part so may be totally wild, are:-
1. that the b series engines were meant to run without the water pump working, ie boiling water...so in order to keep combustion chamber temperatures to a minimum they had them firing at TDC chasing the pistons down the bore..as was said earlier... the explosion, when you ignite petrol takes a few moments to reach full capacity...hence why we have the engine firing before the piston is at the top so when the piston does reach TDC full power is acting on the piston as it goes down, the vacuum advance allows for the higher rotation speeds and the higher piston speeds, so that this keeps happening....ensuring that the engine achieves max power as the revs go up. the B series made big power for those days, a 40's designed normally aspirated engine producing over 200 bhp at 3750 rpm was unheard of and with tests, they got the B81 close to 300 bhp and still relatively reliable, if not fuel economic...and remember this is still a part side valve engine...
2. the combustion chamber shape, wide range of fuel qualities and the fact that it was designed to run at full power and full revs for over 200 hrs or more meant that pre ignition could occur at high engine temperatures, so better to run them firing at tdc.
I sort of remember reading about harmonic balances too, at something like 3300 rpm they were all in conflict or in balance, cant remember but perhaps with too much advance it induced additional in balances .
As I say guesses but I do have a book on the subject and will give it a read to see if I can find out the real reason...