Just to prove how sad I am I have checked through AIP ENR for any other examples of this and find that it seems to be a phenomenon peculiar to the margins of the Manchester TMA - take a look at N57 (formerly A2) in the Blackburn-Burnley area, plus the airspace west of Wallasey and between Huddersfield and Sheffield. There seems to be nowhere else in the UK where airways (other than those passing through TMAs) have a base which is not a flight level.
NS