I overheard the guys at CW in Dorchester who were trying to do it for what sounded like the first time, and it sounded like a bit of a bugger, however it might just be one of those things where it's a gazillion times easier when you know how.
I don't know how many dealers are near you, but in my area I have two dealers within an hours ride, and another within a couple hours ride. Consider phoning around the dealers near you and asking if they have done / willing to do the change for you.
Finally I don't know how permanent you want this to be (for example you might only be doing a few days in france) you could consider using a GPS and changing it to KPH.