The quick answer is that I don't know.What is the official definition of a mile in the U.K.? Are other non-metric units used (8-furlongs, 5,280 feet or 1,760 yards - and how are those defined?).
At a wild guess, I would say that the Imperial units are defined in terms of the SI unit in a way that The National Physical Laboratory says they are, and in turn that is given authority by an Act of Parliament.
Earlier I said:
But the Ordnance Survey, the government agency which makes and sells maps, has produced SI Unit maps since the 1970s.roadsign distances are in miles
Distances on the maps are scaled in cm to km, and contour heights are in metres.
JimB