The UK charter airlines went through a spell of converting charter flights into a scheduled service operation at different times. Some stuck with it - like Monarch - and others like Britannia and Air 2000 ditched the idea after a few years. Others like BIA went out of business between times. In the earlier years, the concept was that a lot of regulatory restrictions used to apply to seat-only sales on charter flights and in many cases, you had to sell accommodation with a charter flight and could only market the flight via tour operator - the airline couldn't sell seats itself. By converting to a scheduled service, you avoided those restrictions. In later years after those restrictions had disappeared in the 1992 deregulation, a switch from charter to scheduled meant you could sell flights via the major distribution systems like Galileo, Sabre, Worldspan etc.
There were other ways around the restrictions though - quite a lot of "seat-only" flights were sold with seven nights' free accommodation included in a tent on a campsite near the airport so that they complied with the rules. If I remember rightly, easyJet still had to comply with a restriction of this nature when they started Geneva-Barcelona many years after the rules within the EU were abolished. Most airlines also had their own tour operators through whom seats could be sold.
Britannia mostly ran leisure routes but also offered a twice-daily 737-200 service between Luton and Belfast for several years, usually running the oldest 737-200 left in the fleet (G-AVRN) on it, which by that stage of its career could just about manage LTN
without a tech stop. Orion, Air 2000, British Island, Monarch all ran leisure routes as schedules and published timetables. Best bet is to visit the event which takes place at Gatwick every year (normally around Easter) with loads of airline timetables and books, models etc on sale and I'm sure you'll find timetables on line.
And as one poster said, LTU published a guide as to which charter flights they ran and the tour operators through whom you could book seats on them. The only document like this which I ever saw from a UK airline was from Air 2000 for its first summer of operations in 1987 which did the same thing, but I've not come across anything like that in the years since.
The ABC World Airways Guide timetables used to list "Scheduled Charter Flights" under the scheduled flights for each citypair if the airline had provided details of the service to be published. [Wardair flights used to appear in the OAGs as full scheduled services.] Of course, any airline could also buy what was effectively advertising space in the pink pages at the back of the second volume of each month's ABC World Airways Guide to sell their wares, and many of the scheduled charter operations could be found in there as well.