Règle correspondante
United Kingdom of Great Britain and Northern Ireland
Practice Relating to Rule 6. Civilians’ Loss of Protection from Attack
Section C. Situations of doubt as to the character of a person
The UK LOAC Manual (2004) states: “Civilians are persons who are not members of the armed forces. In cases of doubt, persons are considered to be civilians.” 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 5.3.1.
The manual specifies:
In the practical application of the principle of civilian immunity and the rule of doubt, (a) commanders and others responsible for planning, deciding upon, or executing attacks necessarily have to reach decisions on the basis of their assessment of the information from all sources which is available to them at the relevant time, (b) it is only in cases of substantial doubt after this assessment about the status of the individual in question, that the latter should be given the benefit of the doubt and treated as a civilian, and (c) the rule of doubt does not override the commander’s duty to protect the safety of troops under his command or to preserve the military situation. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 5.3.4.
Upon ratification of the 1977 Additional Protocol I, the United Kingdom expressed its understanding of the presumption of civilian character as only applicable
in cases of substantial doubt still remaining after the assessment [of the information from all sources which is reasonably available to military commanders at the relevant time] has been made, and not as overriding a commander’s duty to protect the safety of troops under his command or to preserve his military situation, in conformity with other provisions of [the 1977 Additional Protocol I]. 
United Kingdom, Declarations and reservations made upon ratification of the 1977 Additional Protocol I, 28 January 1998, § h.
In 2003, during a debate in the House of Commons, the UK Secretary of State for Defence, Mr Hoon, stated in reply to a question by a Member:
Alan Simpson (Nottingham, South): There have been some civilian casualties for which I am sure that even the Secretary of State would accept that there is a clear line of responsibility. They would include the seven women and children who were killed at a checkpoint and the 15 members of a single family who were killed when their lorry was attacked by an Apache helicopter. Will the Secretary of State tell the House whether current UK rules of engagement allow for such attacks on civilians; whether the rules of engagement for UK troops differ from those of US troops; whether he will place in the House of Commons Library the details of the two sets of rules of engagement; and whether he will confirm that, as has happened previously, any UK troops who were involved in instances of unjustified killings of civilians would be likely to face criminal charges?
Mr. Hoon: We do not comment in detail on rules of engagement, and certainly not on those of the United States. I would be a lot more persuaded by my hon. friend’s observations if, at the same time as mentioning the tragic deaths of seven women and children, he had also mentioned the deaths of the four US marines who were killed in a deliberate car bomb attack, perpetrated by a fanatic. In such circumstances, it is perhaps perfectly understandable – although I am not excusing it in any sense at all – that soldiers who are having to deal with a difficult situation at a checkpoint and who know that four of their comrades have been killed in that way are perhaps reacting in a way that we might not want them to. That is not to say that the accounts that have been given, again, by particular journalists are necessarily the only version of events that we should all accept. An investigation is going on into what went on at the checkpoints, and it is important that we await the outcome of that before judging the facts quite so prejudicially. 
United Kingdom, House of Commons, Statement by the Secretary of State for Defence, Hansard, 3 April 2003, Vol. 402, Debates, col. 1085.