Usually cruise related - Fred Olsen from memory - and I think normally whole plane charters (so not bookable) with passengers returning from another port (so appears one way but pax actually return from another airport).
Again, from memory so not 100%, I think one lot of pax board at port A and disembark from port B, while another lot go from B to A. Might be some empty sectors positioning into regional airports in the UK, but the empty long haul sectors will be at the start and end of the season.