Fly it to Dubai instead and plug into the EK network, possibly do an onward flight to London from there as a code share with Emirates and carry some of their passengers for a change.
The extra fuel used compared to uplifting en route will require a higher airfare and oil prices may not stay at their current low levels.
18 hours would need a low density seating configuration to be bearable in economy, possibly the 8 across one which it was originally designed for.
The A350 in 9 across is still quite comfortable as is the B777. This route would have mainly British and Australian passengers onboard, countries which rank among the worlds highest rates for obesity.
I would pay extra to have a stop on the way over, 18 hours in the middle seat sandwiched between a couple of porkers would put me off flying forever.