https://maccmodels.co.uk/batometric
The link above is to a chart giving metric dimensions for BA threads. The "close BA to metric match" I referred to is 0BA which is almost exactly a M6-1.00 thread, others aren't as good. The company (UK location) sells both BA fasteners and taps/dies.
Another source for BA fasteners, taps and dies is
https://britishfasteners.com/ located in New York State.