Just did a quick bit of homework and it probably wouldn't matter using whitworth or UNC because they are both the same diameter and pitch, just slightly different thread shape and angle. I'd find it a bit strange if they were actually made with a Whitworth thread beacause it is rare to use whitworth these days and the international standard for the mounting hole in the camera is 1/4 UNC. Having both UNC and Whitworth in the same system would be a bit silly I reckon.
It should be all metric anyway
I use a 3/8 UNC bolt to secure my Manfrotto ball head no probs
