These charters have been done by Qantas for a couple of years now. They are for a cruise ship company operating in the Mediterranean and elsewhere and use one of the 747s that normally spends all day at Heathrow. They tend to go to a different point each time meeting the ship. Obviously a whole shipload of passengers is appropriate for the 747s capacity, and it's a marginal time operation but one that suits an aircraft otherwise parked up from early mrning to late evening (of which there are quite a few at Heathrow each day).
Here's the original plan for the rest of August:
Sat 14 Aug - QF 6048 Copenhagen
Fri 20 Aug - QF 6066 Barcelona
Mon 30 Aug - QF 6070 Athens
They tend to leave about 08.00 local and return in the afternoon.