I need help with a headset.
 

Ive aquired a British? frame to build a singlespeed.
I have stripped down the threaded integral / internal ? headset, and want to fit some sealed bearing if possible.
Not sure what to get that will fit the bearing seats in the steering tube.

That is just a standard external headset, and the "seats" are pressed-in bearing cups. The simplest solution is to buy loose ball bearings to rebuild it.

Make sure the races are nice and smooth and free of pitting first. That will ensure your headset will run smoothly and free of play.

Dave

Size of ball bearings?
 

Is there a standard ball bearing size for these bearing seats?

No real standard, but the most common size is 5/32".