Pat,
The Northumberland Marriage Index includes only Anglican marriages simply because, in theory, they were the ONLY marriages that could have taken place.
Under English/Welsh law, ( Marriage Act of 1753) the ONLY legal way to marry was in a church of the established faith = Church of England = Anglican.
People of Jewish or Quaker faith were exempt from this legislation.
Catholics, Presbyterians, Methodists, etc, had a stark choice.....
*Marry in an Anglican Church( to them , the "wrong" church)
*Not marry ( any children thus illegitimate, bringing complications including inheritance etc)
*Marry outside England/Wales (e.g. Scotland)...
That is not to say that "secret" marriages did not take place in "non-conformist" churches (baptisms in other-than CofE, were not forbidden), but as for as the state was concerned, any such marriage was null.
The Marriage Act of 1836 and other bits of legislation, brought in sweeping changes, effective from July 1837, including the civil registration of BMD, allowing the "non-cornformists" to marry in their "own" church, and non-denominational marriages in a Register Office.
So, in theory, before July 1837, the ONLY marriages were CofE marriages.
Many English crossed into Scotland to avoid marrying in the "wrong" church. All did not necessarily marry in the "right" church.
For example, some marriages were conducted in the toll booth on the Scottish side of the bridge over the River Tweed at Coldstream.
Further to the west, there was marriage facilities at the well-known Gretna Green. And due north of Berwick-upon-Tweed marriages were conducted in Lamberton Toll.
These sites were set up by Scottish entrepreneurs to cater for the "trade".
A Berwick newspaper mentioned how the daily train pulling in to Tweedmouth from Newcastle, was swamped by Scottish marrying men, looking for business.
Most of these "Irregular" marriages, although religious, were non-denominational.
Records of some, but not a lot, of these marriages exist.
Newcastle Library has an index of "Coldstream Bridge" marriages, but they are in poor shape, some missing dates, some giving individual brides and grooms, but not as "couples". I will have a browse for your P/P.
And if the couples were not in a hurry and had sufficient cash, they could have avoided the "quickie" marriage and gone deeper into Scotland to a church of their own religious denomination.
Also the English and others, went to Scotland to marry, because Scotland had less tough rules on need for parental permission. This type of marriage sometimes called "runaway marriage"
Although this is long, it is only a summary of the whole picture. But lots on line about.
Michael Dixon