Must take exception to your statement that "most regional airports" are struggling with TA traffic and the mis-leading statement that only 25% of LHR transit traffic has MAN as a destination, inferring that MAN does not attract as many TA travellers as EDI.
MAN has 10+ scheduled flights to the US most mornings on scheduled airlines (excluding Thomson holiday flights)
NY : United x1, American x1, Thos Cook x 1
Orlando : Thos Cook x 1, Virgin Atlantic x1
Atlanta : Virgin Atlantic x 1
Philadelphia : American x 1
Las Vegas : Virgin Atlantic x 1
Miami : Thos Cook x 1
Chicago : American x 1
Houston : Singapore x 4 weekly
Both Virgin and Thos Cook have new West Coast destinations starting in 2017, and both usually increase their schedules during the summer season; also, it is anticipated that SQ will go daily to Houston sometime next year following the introduction of the A350 onto the route early in 2017. Added to this, there are seasonal services to Canada on a couple of airlines, and numerous services to the Caribbean, both seasonal and year-round.
Now please explain to me why should anyone heading for MAN need to transit elsewhere?