UPDATE 1-AIRSHOW-Emirates says to order more A380s at Dubai airshow
* Boeing seen launching 777X with around 250 orders, 4 buyers
* Airbus fights back to avoid rare air show defeat
* Boeing says union vote on 777X assembly won't deter launch
* Emirates airline seen in talks to buy 20-25 A380s -source
* UK's Cameron pays lightening visit on eve of show (Adds Emirates chairman comments)
By Tim Hepher and Praveen Menon
DUBAI, Nov 17 (Reuters) - Dubai flag carrier Emirates will buy more Airbus A380 superjumbos, the airline's chairman said at the opening of the Dubai Airshow on Sunday, adding total plane orders would exceed the $18 billion agreed at the last event in 2011.
The airline is the largest customer of the A380s, with a current order of 90 jets.
"We will order A380s," Sheikh Ahmed bin Saeed Al Maktoum, the chairman of Emirates, told Reuters. "Yes, for sure. We (Emirates) will exceed" the previous total round of orders. Continued...