So. (Apologies again if this is wrong, I still don't think I get it, and it's not through your lack of explanation, something is just not clicking with me, and contrary to my complete inability to grasp this thread, I'm not a complete dimwit usually)
Northern. Large area of UK, under which all regional dialects, are Northern accents.
Mancunian. Specific (small) regional dialect. Mancunian accent.
RP. Non regional dialect. (As many of the regions in the South have regional dialect, not RP). RP accent.
You don't have to fall under a regional dialect. You do have to fall under an accent.
If you are Southern England, and don't have any regional dialect, you are RP.
(Tell me that's fucking right, for the love of God)