for servicing, a normal competent (thats the hard part) garage should be able to do everything..
William Rutter Motor services in bedford are excelent, and are about 100 yards from John Woolfe Racing, so you have the advantage of having an excelent parts source right next door.. TBH they are all i realy use now, and have never put a foot wrong. One of the guys there owns a late 80's caddy, and the guy that owns the place has had quite a lot of expirence with limo's so knows late model fords well.
reasonable labour prices too..
The main ford dealer in Bedford (when it was in its old ownership) were happy to help with US ford, the only stipulation is that you provided parts. as they are now part of priory group, i dont know if this is still true, but they used to carry microfishe for the explorer, long before they were imported officialy..
i've heard quite a few horror stories about WIM (including one completely botched job i've seen first hand) and they aren't cheap, avoid like the plague.