Could it be due to the fact that London Mil still exists at West Drayton (Everything East of the "Alphas") whilst Swanwick Mil is somewhere else?
Regardless, I personally agree that units should call themselves what it says on the tin. However, I will be interetsed to see how this is handled when nPC takes everything North of Manchester. Will they call themselves Scottish even though they are working in both the Scottish and London FIR/UIR?