Assuming you have a Series 1 or 2 F-frame, AFAIK any standard 68mm BSC thread bottom bracket will fit without modification. Apparemtly the bottom bracket shell on these F-frames is offset to the left, so I added two 1.5mm Sturmey Archer sprocket spacer washers onto the drive side bottom bracket cup before fitting. However, I’m not sure how critical this is.
I used a 113mm BB which was fine for the old SR Chainset I used.
I don’t have one myself but I understand that Mark 3 Moulton’s have a much wider bottom bracket shell (76mm?) and used Raleigh’s proprietary threading, so they do need modification in order to fit a standard BB.
Hope this helps