MANFOD
I used to check the seat maps for MAN, BHX & EDI regularly but not for around a year now. I found both MAN and BHX strong in business well in advance of the departure date as well but as you say we have no idea how many were full fares and how many were upgrades.
However one area where the Manchester service beat the BHX service hands down was 45 premium economy seats. A few months prior to departure BHX was always light usually around 5-10 seats taken whereas Manchester usually had around 20-30.
Again I suppose not particularly a reliable source of information without knowing the actual fares paid but this was a regular pattern at times during 2016 and whether this aspect is keeping the Manchester service going we will probably never know.
As for fares, I only used to check economy and BHX/EDI/MAN were pretty similar outside of the peak months with few bargain economy seats outside of January and February.
It is one of those services that you would not be surprised to see upgraded or surprised to see it go and despite the 757 still having a few more years left it won't be too long before something happens.
As posted on the Flybe thread it appears Luxemburg is getting loaded for winter.
Pete