You'd find that Easyjet would have to charge around the same as the others due to may factors such as cost of living and doing business in Australia (factor in importing most things and paying for aircraft maint vittles in USD etc) from being so far away from "the action" of the US and EU down here. So, when exchange rate (currently at AUD1.51:1BPS and everything factored in it costs AUD$200 more for the whole trip or, should you prefer to factor the actual CPI's of the two countries (1.22 for Australia and currently 100.13 for the UK - look it up yourself:
Cost of living index by Country ) you'll find that in
real terms you're paying around AUD35 bucks each way (20 quid) more for a seat on a three quarters full aircraft because maybe, just maybe the load factors may not always be what they are in Europe due something so trivial as population base... We do pay a little extra for our fresh air and wide open spaces, high wages and equally high taxation.
You do the math and have yourself a really lovely day. We'd like to see you down here really soon but please leave that "apparent" chip on your shoulder back there and arrive with an open mind.