B44 & B50s have rubber sealing rings on the rocker spindles. I haven't looked inside a BSA for many years, and too much rough red has destroyed the memory, but if pressure feed to the rockers leaks here, the build up goes down the valve guides.
Just a guess. You can remove the rocker box studs which will allow you pull the rockerbox and or cyl head without pulling the whole motor out of the frame each time. Just don't do this too many times, or they will come loose in their threads.
Mike