Problem is a business class airfare LHR - SYD is the same as LHR - JFK even though it's three time the distance. How many trans atlantics could done in the time it takes to fly to Australia and back. Flights across the pond don't need a crew change en route either.
There is serious competition on the kangaroo route at all levels e.g.
cheapest price, China Southern or Thai
premium service, Singapore Airlines or Cathay
best connections, Emirates (Newcastle - Brisbane one stop)
stop over en route, EK, SQ, CX, TG
Going to Australia BA won't be the best or the cheapest, where as on the North Atlantic the main competition are airlines from the US with equally high prices and poor service. BA can also offer good connections from their extensive European network to the USA via London.
There are simply too many alternatives between the UK and Australia for BA to compete. It's not the 1970s anymore.