George,
The only "legal" way to marry in England and Wales in that era, was in a
" Parish Church", that is; Church of England.
So the choice for Presbyterians ( and Methodists, Catholics, etc) was to marry in an Anglican church, just not marry or marry outside England/Wales. e.g. Scotland.
No similar restrictions with baptisms.
First baptism for their children was presumedly in a church within the bounds of the Parish of Bolam ? ( within which lay the community of Belsay)
So they may have previously married at Bolam Parish Church ( St Andrew's) or just popped over the border, not for a "cross-border" or "Irregular" marriage, but for a regular one in a Church of Scotland=Presbyterian Church.
Michael Dixon