The freighter market at both LHR and LGW airports is overwhelmingly influenced by the scarcity (and market value) of runway slots at these locations. This is a much lesser consideration at MAN, where the freight industry could more easily avoid peaks in traffic demand for the runways.
Freight flights into LHR have boomed recently, whereas LGW has seen nothing, so there's more to it than slots. DSA, PIK, BHX, STN have seen increased traffic, but EDI, GLA, NCL, LBA, LTN, BRS, CWL haven't. So those airports with existing infrastructure to handle large freighters have seen more, and those that don't haven't - not rocket science is it?
It remains to be seen if this is a shift in the cargo market or just a blip.