I too am planning to use theBritish Horse and Dragoons to bolster my slowly growing British SYW army. Quite some years ago I converted some Revell SYW Austrian Dragoons with a head swop and a lot of carving into British Light Dragoons long before the real thing became available. But still very pleased with my own efforts in this case.
Not looked up the history of the regiment but the recently (about 2 years) deceased Duchess of Alba had the surname "Fitzjames". Was this one of those Irish regiments that joined either the French or Spanish armies since war with Britain was always a possibility?
Pretty much. It seems to have been an Irish regiment raised by James II in France in the 1690s after his exile, organised and equipped as a regular French cavalry regiment, and was shipped over to Scotland in 1745 to support the rebellion. There's some more info about the regiment here: http://www.kronoskaf.com/syw/index.php?title=Fitz-James_Cavalerie
I just looked up and it seems like the colonel was one Charles de FitzJames, James II's grandson through an illegitimate son (hence the surname FitzJames), from whom the Duchess of Alba was also descended, making Charles her great x 7 uncle or something like that.