I believe for the Scandinavian charters the Sunday afternoon group, which alternated between Luton and Stansted, were holiday charters to hotels in London, popular for one week shopping etc trips at a time of no LCCs and scheduled operators at Heathrow only pricing for business travellers. The midweek flights were for the student travel organisations (cheaper charter rates) and more random in their operations.
Tjaereborg rings a bell as one of the main charterers
Tjaereborg were the proprietors of Sterling Airways, and they did a very wide range of operations - they did Caravelle charters from Copenhagen to San Francisco (3 fuel stops along the way). Kar-Air used Finnair aircraft as, like a number of the mainstream European airlines' charter subsidiaries, they had no fleet of their own. Flying Finn in much more recent times had a go at being an LCC themselves, with MD-80s, and even operated as such from Helsinki into Stansted for a short while around 2003, but didn't last.