I suspect that as well as rail taking passengers from the MAN-LHR route, there is growing tendancy to make connections at Schipol/CDG etc rather than LHR, which frankly is just too unpleasent in comparison.
The awful summer may mean higher bookings next year, as the pattern of bookings holidays at home because of improved summer weather is broken.
As for the US flights, I know a lot of people who won't travel there anymore because of the security hassle and real/perceived rudeness and dislike of the way they are treated both in the UK and the US. If the figures really are falling with a 2$ to the pound exchange rate, then that is not a good sign. JFK & NYC in general seems over-serviced from MAN anyway.
If you do go via ORD avoid it like the plague in the winter months, it grinds to a halt for days on end when the snow and storms come in.