Darren,
I think you are getting confused by a name. BSF means what? There is nothing to compare it with except BSW. It is just a size, pitch and angle. The chart I showed proves it.
The metric threads you normally use are called metric coarse, only because they are more coarse than metric fine, but I don't consider them very coarse at all. It is just a name.
Your mini lathe wouldn't stand a chance, not only wouldn't it fit diameter wise, it would most probably burn it out just trying to turn a lump of metal that size.
I can make backplates for you, but it will have to wait until my workshop has been cleared of all jobs and tidied up, it could be a few months. But it would mean you removing your lathe spindle, I wouldn't even attempt it without. Just a couple of thou out and it wouldn't fit or be too sloppy a fit. Cast iron is the only material I would use, again, I wouldn't attempt it with anything else. Just get a quote for material, and be sitting down when you read it. Expect around £30+ per inch for 8", around £10+ per inch for 5".
It all depends on what sized chucks you want to fit, 5" would be reasonable, 8" might be a second mortgage type of job, especially as it seems the S&B has a longer than normal thread, so would require a longer length of material. It would need a length from the back datum face to the end of the threads, plus 3/8" to allow for machining and thread front recess.
John