David Cameron visits troops in Afghanistan

A post-2014 mission could involve around 8,000 American and 6,000 allied troops.

The U.K. has committed to leaving a small number of advisers to Afghan's Ministry of Defense and trainers at the national officer's academy in Kabul—and those plans could be in jeopardy if a deal is not signed.

“Clearly Britain wants to continue playing its role,” Cameron told reporters.

He noted that “the big drawdown” is now taking place and praised British troops for helping to train the Afghan National Army.

Cameron toured Camp Bastion, having breakfast with troops and taking in a soccer training session between Afghan soldiers and British troops.

He then then took a Chinook helicopter to one of the last four British forward operating bases.