Not sure, but it also applies to other cars listed in the S.U. catalogue for that era (early 70's) including M.G's, Rover, Other Triumphs, Mini's. I think it was to do with the introduction, harmonisation and compliance of the "UNECE regulation 83 related to "emission of pollutants according to engine fuel requirements" - words stolen from wiki!
If this is the case then I think ECE stands for Economic Commision For Europe - one of their remits was harmonization of vehicle regulations:
https://en.wikipedia.org/wiki/World_For ... egulations
My guess is B.L. had a program to get its cars compliant/certified etc to the regulations in terms of emissions and these are the cars/models denoted by E.C.E. in the SU book.
Of course I could be totally wrong !! But I can find no reference to its meaning in any of my old SU books or any where else for that matter.
Roger