This situation is clearly nonsense, however, they are not private roads. Technically the civvy police could do their own checks on RAF Stns, but they leave the 'self-policing' (excuse the pun) to the RAF/MoD authorities. You can get done for booze driving on Stn! The point here is that it is sad that the RAF plod have nothing more satisfing to do.
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Which bit of dire retention problem are THEY finding hard to understand...