I use EN16T for spindles (fork, wheel etc). Threads should be BSCy not WW. I think Ariel used 1/2" 20TPI as opposed to the standard 26TPI for half inch, but guess that doesn't really matter so much.
Ariels - 1913 TT Model, 20 Roadster, 28 Model D, 30 Model B, 38 VH, 52ish VH
The Offroad Fleet - Fantic 200, Bultaco Pursang, Beta Rev 3