One of the other things you have to bear in mind is that on occasions the the two main Birmingham churches had over 200 marriages taking place in one day. Since the clergyman conducting the marriage could not possibly have time to put all those details into the register, it was left to the parish clerk to take notes and complete the register later. So ages, "of this parish", even occupation, and certainly some addresses need to be viewed with suspicion.
Mary from Little-Bogwit-in-the-Marsh wants to marry John, but she's under age and she knows that her father would never give permission. Get married in a big city where no one will scrutinise the marriage too much and father won't find out at least until it is too late. She probably never had any intention of staying in her little village anyway. So a few lies, white or otherwise, will result in the marriage that John & Mary so desire. This did happen quite a lot.
So you need to find a link somewhere - collaborative evidence. For instance, when my LONGMOREs moved to Birmingham from Darlaston, how could I be sure that I had the right family? There were several families with that surname in both areas. In my case, for generations, back to the 1600s, a member of my LONGMOREs was christened Lambert, Lambeth, or Lamberth in almost every generation. When it started appearing from 1811 in my Birmingham LONGMOREs, I knew I had the right family because it is such a rare forename.
So if you suspect that a family with the same name in, say, Essex, might be linked to your Brummies, you need to do a detailed check on that parish, and possibly surrounding parishes, going back several generations if you are able, to see if any sort of pattern arises.
Maurice