I just looked at three or 4 pumps and the thread is fine, so I would not have thought BSP. I did not screw a bolt in but it does look like 1/2" UNF
Jag senders can be coarse or fine thread, so I don't know if Rover switched between the two as well.
Beware also the new ones are all over the shop in resistance, I would try to find a working OEM sender.