Thanks very much for all the replies chaps. I should perhaps have made clear that under no circumstances was I planning to repaint anything, as Majorsmith's painting skill far exceeds my own; they will remain the best-dressed force at my disposal, as befits their status as soldiers of the Queen. Thus, they will remain a Line Infantry regiment, from England as that seems easier, and almost certainly to start with, from Trumptonshire (which is more or less where Wiltshire is I think). So they will for the time being, be the Trumptonshire Regiment, following the merger under the Childers reforms of the North Trumptonshire Volunteer Regiment and the Duke of Melchester's Own Regiment of Foot.
Plynkes, the pic of the early-20th Home Service Undress chaps in their natty caps and white jackets is amazing. Very 'what if' indeed. My suspicion is that there are Ruritanians just out of view...
Thanks again guys, this certainly has been an education.