That wouldn't be a common charter operation, which doesn't suit that sort of route as the need for multiple crews along the route, and balancing return loads doesn't work well for a low cost operation. Plus ad-hoc charters are not normally permitted at London Heathrow. Charters have also long been restricted very much by the air service agreement between the UK/EU and Australia.
However, you were there and you rode it ! And having said that, at that exact time, for a couple of years, a charter operator called European Aviation had bought up several of the old British Airways Boeing 747-200B fleet which had been retired, and operated them over 2002-5 all around, economy only, charters from Asian countries to Mecca etc. Although based in Bournemouth their 747s, if operating work from the UK, generally ran from Gatwick. Going through Colombo is a good halfway point for aircraft with medium-long range capability to do a one-stop hop from London to Sydney. Going through the Gulf has cheaper fuel but the long hop on from say Dubai to Sydney was really beyond aircraft of the time which cheaper operators had.
The photos are not a great help because each aircraft seems to have had a different livery, sometimes more than one in their short service. But the colours might ding a bell. Although based in the UK Paul Stoddart, millionaire owner of the airline (and a Formula 1 racing team) was Australian
https://en.wikipedia.org/wiki/Paul_Stoddart and had a failed attempt to set up an associated Australian airline called OzJet at the same time.
https://www.jetphotos.com/photo/9061658
https://www.jetphotos.com/photo/9219157
The only "regular" charter operator of the era that went to Australia was Britannia Airways, with Boeing 767s, who ran winter (in UK) off-season flights there, maybe once a week. They commonly ran from Gatwick as well.