Nice wind up Bermuda & Skippy - but that one's been done to death before.
Anyone who uses EK, SIA & LH will know J class is often overbooked.
It has nothing to do with "demand" but only to do with channelling all available traffic via LHR to make up their numbers down there as they can't provide the yield they need for using that high cost airport.
My experience shows many BA long haul flights, with less than 150 pax, sometime less than 100 & J class almost empty. The regions are turning their backs on LHR & using other euro hubs.
I don't think anywhere is going to grow these next couple of years, in that respect MAN is no different to LHR. That goes for the lo co sector too, whether it be MAN or STN / LGW