Go for the later one. In my experience BD will move heaven and earth to accommodate late connecting pax from EDI or GLA. Twice I've arrived into MAN from EDI well after boarding has started for the ORD/IAD and twice I've been offered an airside car or an escort to the gate. They fell over themselves to ensure my bags were loaded, too. That you're all in C will also help matters considerably.

One thing worth checking: does BA have a GLA-MAN flight at a similar time, should the BD flight go tech or get cancelled? A useful fall-back.