* Virgin Atlantic seen ordering A350s, cancelling A380s
* Germania deal could be worth over $2.5 bln at list prices
* UK PM Cameron expected to finalise military deals - sources
* Boeing expected to relaunch smallest version of 737 MAX (Adds Airbus and GoAir in talks; Boeing comments on demand)
By Tim Hepher
FARNBOROUGH, England, July 10 (Reuters) - Airbus is set to announce a key order for 8-10 of its biggest twin-engined model, the A350-1000, from Virgin Atlantic after beating Boeing to a scarce order for big jets as economic and political uncertainties overshadow the Farnborough Airshow.
The order, worth some $3 billion at list prices, could perk up Monday's opening of an event clouded by geopolitical uncertainty, including Britain's decision to leave the European Union and concerns over the state of the global economy.
British Prime Minister David Cameron will attend the opening day, where he is expected to finalise multi-billion dollar deals with Boeing to buy nine P-8A Poseidon maritime patrol planes and to upgrade 50 Apache helicopters.
Industry sources said Airbus's A350-1000 deal with Virgin Atlantic provided a shot in the arm to a 366-seat model promising greater efficiency but whose sales have slowed as Boeing develops a larger version of its 777, prompting a game of leapfrog between the only two makers of long-haul jets.
Airbus declined to comment and Virgin Atlantic was not available to comment.
Airbus and Boeing have enjoyed years of strong demand, helped by rising airline travel particularly in Asia and the Middle East and new, more fuel-efficient planes.
But with the industry's order backlog standing at a record 13,500 planes at the end of 2015, or 9.6 years of production at current rates, analysts are worried that economic risks - from slowing growth in China to Britain's move to leave the EU - could see orders dry up and some even cancelled.
Larger, more expensive twin-aisle planes are likely to be most at risk. Indeed, industry sources said Virgin Atlantic, which has long deferred an order for six of Airbus's A380 superjumbos, looked set to finally cancel it.
The head of Boeing Commercial Airplanes, Ray Conner, said the company was facing slower demand for wide-body jets and would be vigilant about matching supply to demand, though it was too early to say whether it would again cut output of its 777.
SINGLE-AISLE COMPETITION
Separately, industry sources said privately held German airline Germania was close to placing an order with Airbus for around 25 A320-family jets, worth over $2.5 billion.
That deal for Airbus's top-selling single-aisle plane, if confirmed, would also be a disappointment to Boeing, which is trying to wrestle back such customers after losing market share.