We have the same thing with National Pipe threads here in North America. We have NPT and NPS....tapered and straight. Tapered will tighten up after a few turns, and straight will go through just like a nut and bolt.
A quick google will give all the specs to cut the BSP parralel threads. Even a few suppliers of the taps and dies, but I would go with a lathe myself. When you get to these larger sizes, by the time you buy the taps and dies, then the handles to match, you are well onto your way to buying a decent used lathe.
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