A contract to begin work on the first two British P-8
 
aircraft has been awarded by the US Naval
 
Air Systems Command. 
The contract is expected to be complete by the end of July 2017.
In November 2015, the UK announced its intention to order nine P-8 aircraft as part of the Strategic Defence and Security Review 2015. The aircraft are to be based at RAF Lossiemouth, Scotland and be used to protect the UK’s nuclear deterrent and new aircraft carriers.
The P-8s are also to perform search-and-rescue missions and conduct overland reconnaissance.
In March 2016, the US State Department approved a proposed Foreign Military Sale to the UK for up to nine P-8 aircraft and associated support, at an estimated cost of $3.2 billion. The Royal Air Force plans to operate the P-8 with US weapons initially and possibly transition to British weapons later. It is unclear whether the UK will have access to future ground-surveillance capabilities being developed for the P-8 but many expect it will.
The UK government has revealed that a recent contract includes the purchase of unspecified US weapons, sonar buoys and “other consumables”.
According to the contract announcement released by the Department of Defense in the US:

“The Boeing Co., Seattle, Washington, is being awarded a $68,409,026 modification to a previously awarded advance acquisition contract (N00019-14-C-0067). 
This modification provides for long-lead parts and efforts associated with the manufacture of two full-rate production 4 Lot 8 P-8A Multi-mission Maritime Aircraft for the government of United Kingdom under the Foreign Military Sales program. 
Work will be performed in Seattle, Washington (82.6 percent); Baltimore, Maryland (6.2 percent); Greenlawn, New York (4.2 percent); North Amityville, New York (3.5 percent); and Cambridge, United Kingdom (3.5 percent), and is expected to be complete by July 2017.”
The government had earlier admitted that the British P-8 Poseidon maritime patrol aircraft will not, at first, feature British weapons.
Asked by Mr Kevan Jones(MP for North Durham):
“To ask the Secretary of State for Defence, whether the P-8 Poseidon will be capable of carrying UK Storm Shadow and other manufactured munitions.”
Answered by: Philip Dunne:
“The Department intends to bring the P-8A into service without significant modification to ensure the delivery of operational capability as soon as is practicable. There are no current plans to integrate Storm Shadow or other UK manufactured weapons onto the aircraft.”
The P-8 conducts anti-submarine warfare, anti-surface warfare and shipping interdiction along with an electronic signals intelligence role. This involves carrying torpedoes, depth charges, SLAM-ER missiles, Harpoon anti-ship missiles, and other weapons. It is able to drop and monitor sonobuoys.
It is designed to operate in conjunction with the Northrop Grumman MQ-4C Triton Broad Area Maritime Surveillance unmanned aerial vehicle. The aircraft has been ordered by the Indian Navy as the P-8I Neptune, and the Royal Australian Air Force.