Lethal Force

The UK's policy on using force overseas is clear. It will be in line with applicable UK and international law, and bounded by UK rules of engagement, targeting and operational directives. The same laws are adhered to whether the method used is an unmanned and remotely piloted air system (also referred to as drones), a piloted aircraft or any other weapon.
Under international law, the UK has an inherent right to self-defence against both armed attacks and imminent threats of armed attack. Where a direct and imminent threat to the UK is identified, and there is no other effective option, the Government reserves the right to take lawful action in self-defence to address the identified, imminent threat to the UK or UK interests and to report to Parliament after it has done so. I know that the Government made its position clear to the Joint Committee on Human Rights during its inquiry into the use of drones.

The UK expects others to follow the same international laws when it comes to the use of military force. This applies to the use of armed drones, as to any other weapon, including when used against terrorist targets. 
One purpose of the Ministry of Defence's Joint Doctrine Publication on Unmanned Aircraft Systems (UAS) is to increase transparency and understanding around the use of military drones. The doctrine applies to all uses of UAS across Defence at the operational level and it guides operational commanders and planning staff in understanding the terminology, tasking and employment of military drones.