I have worked in the logistics sector for many years and in particular airfreight at Manchester, Leeds/Bradford and Heathrow. I echo the previous comments that freight doesn't care where it lands. In my younger days at Manchester in the mid 70's onwards we had freigters every night to Dublin, Amsterdam, Frankfurt, Zurich etc, all now gone replaced by trailers to said airports.
The vast majority of cargo arriving/leaving by air at Manchester is carried in the belly of cargo aircraft, so it could well be argued (current situation permitted) that increasing passenger flights is the driving growth for cargo at Manchester not the occasional ad-hoc charter. For scheduled freigters, there are other airports best suited to handle them in the group and that makes economic sense for MAG.