I think that for the past couple of years at least (and it may have been longer), many of the BY services from HUY have been operated by the aircraft based at LBA, hence the sight of BY 762's at HUY during early summer 2003/4 when this aircraft was being used at Leeds, before reverting to B757's from June onwards. The times of the flights also suggest this as on certain days the aircraft would leave LBA early morning and not return until late at night, with HUY having had a mid-afternoon BY service.
Not sure about MYT, but again I think that the LBA based aircraft used to do some of the winter 'W' services, such as TFS. G-PIDS was the usual B757 based at LBA in winter 2003/4 and was also regularly seen at HUY during this time. Could be that a MAN based aircraft may do some of the services too and in the future.
From what i have seen, aircraft operating 'W' patterns often keep close to airports that the aircraft is based at, presumably as it makes crew changes/travelling much shorter and easier.