AndyH52,
Manchester tweeted their spare capacity not so long back, that is where I got the 40% from, infact recently they have been mentioning being able to handle 50m pax now (currently 22m+), which as you correctly point out mention at times would seem impossible, but there are large chunks of the day where much more could be handled.
As for flying from your local airport, would that be Liverpool ? So we'll use new slots at LHR to add more domestic air traffic, start LPL-LHR flights when MAN just 30 miles away has plenty of shuttles already, and I am sure LPL is also well connected by trains, to London.
MAN is just up the road from LPL, and in the interest of the NW / North I'd rather support growth there than supporting a new runway at LHR just so LPL can get shuttles to LHR.
No doubt LHR can also have a JFK flight every half hour with a new runway, with more flights there in a few hours than from the rest of the country combined, and ironically fairly well populated by folk connecting on shuttles, that would rather quite possibly fly direct.
Hopefully the politicians will remain scared to death of making the call on their watch, then maybe the Northern powerhouse may stand a chance in terms of aviation.