So I had a look here:
https://en.wikipedia.org/wiki/List_of_British_postage_stamps
and found no trace of any stamps referencing the (former) Metropolitan Counties. Now of course a Wikipedia article could be wrong but I suspect there are enough die hard philatelists around that we can trust this one to be accurate. Your assertion that the Post Office is somehow responsible for negative images of the Greater Manchester area is therefore a figment of your imagination. And surely the overspill estates housing large numbers of City fans are Wythenshawe (M22/M23) and Langley (M24).
Also note that the Post Office did issue stamps commemorating both the UK's joining of the EEC and the first direct elections to the European Parliament. It would be perfectly appropriate then to issue stamps to commemorate something as impactful as Brexit!
Maybe they should wait until it actually happens rather than be at the behest of pressure from those frustrated that its progress doesn't look like it was such a good idea.