This is a quote from the Wikipedia entry for the former Lincoln St. Marks Station, and I have emboldened the key part
Who would have decided that the long established direct London-Grimsby/Cleethorpes service would not be a part of any post-privatisation franchise?
Were there another long established services on the network brought to a sudden end at privatisation?
The main route for London - Grimsby / Cleethorpes services was via Peterborough, Spalding, Boston, Firsby & Louth, most of which closed in 1970, and services were not exactly frequent. The route via Newark Northgate only became possible after building of the curve onto the Lincoln line in 1965.
In the 1955 timetable, there was just one train daily each way between Lincoln Central & London Kings Cross via Leadenham & Grantham. There were 2 each way between Grimsby or Cleethorpes and Kings Cross via Boston (with one extra on Summer Fridays / Saturdays).
The 1975 timetable shows 3 trains each way between Cleethorpes & Kings Cross via Lincoln St. Marks.
Some other services that ceased pre- or post-privatisation include Euston to Blackpool / Shrewsbury, Liverpool - Cardiff'
Plus what remained of some Summer Saturday services like Newcastle - Yarmouth, Derrby - Yarmouth, Leeds - Skegness & Yarmouth, Euston - Aberystwyth & Pwllheli, etc.
