Related Rule
United Kingdom of Great Britain and Northern Ireland
Practice Relating to Rule 67. Inviolability of Parlementaires
The UK Military Manual (1958) provides: “Whilst performing their duties, and provided that their conduct is correct, [parlementaires] are entitled to complete inviolability.” 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, § 391; see also § 498 (protection of cartel ships).
The manual stresses:
When a white flag is hoisted the other side need not necessarily cease fire.
Fire must not be directed intentionally on the person carrying the white flag or on persons near him. If, however, the persons near a flag of truce which is exhibited during an engagement are unintentionally killed or wounded, no breach of the law of war is committed. It is for the parlementaire to wait until there is a propitious moment, or to make a detour to avoid a dangerous zone. 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, §§ 395 and 396.
The manual further states: “In addition to the ‘grave breaches’ of the 1949 [Geneva] Conventions … the following are examples of punishable violations of the laws of war, or war crimes: … firing on a flag of truce.” 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, § 626(d).
Furthermore, the manual states: “The parlementaire should be permitted to retire and return with the same formalities and precautions as on his arrival.” 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, § 411.
The manual also states:
The number of persons who may accompany the parlementaire to the enemy’s line, unless special authorisation for additional ones is given, is limited to three; a trumpeter, bugler, or drummer, a flagbearer, and an interpreter. These are entitled to the same inviolability as the envoy himself.
In modern warfare the parlementaire will presumably be an officer in an armoured vehicle flying a white flag, accompanied by his driver, wireless and loudspeaker operator, and interpreter. 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, § 400, including footnote 2.
The UK LOAC Pamphlet (1981) states:
The white flag, or flag of truce, indicates no more than an intention to enter into negotiations with the enemy … The party showing the white flag must stop firing and if so the other party must do likewise … The flag party may not be attacked and on completion of its mission must be allowed to return to its own lines. The [1907 Hague Regulations] provide for the flag party to consist of the envoy, flag bearer, interpreter and trumpeter, bugler or drummer. In modern warfare the latter may be replaced by a radio operator and the flag party may well travel in a vehicle flying the white flag. 
United Kingdom, The Law of Armed Conflict, D/DAT/13/35/66, Army Code 71130 (Revised 1981), Ministry of Defence, prepared under the Direction of The Chief of the General Staff, 1981, Section 4, p. 16, § 10.
The UK LOAC Manual (2004) states:
10.4. … Whilst performing their duties and provided that their conduct is correct, parlementaires and those who accompany them are entitled to complete inviolability. This means that they may not be attacked or taken prisoners of war and must be allowed to rejoin their own forces at the end of their mission. Inviolability of the parlementaire does not mean that all hostile activities of the opposing forces have to cease. They must cease to the extent necessary to ensure the safety of those involved in the negotiations.
Vehicles and aircraft
10.12. Vehicles and aircraft used by parlementaires have the same protection and may not be seized. These provisions extend also to naval vessels, known as cartel ships. The personnel of such ships have the same rights and obligations as parlementaires. Cartel ships may also be used for the exchange of prisoners of war. Such vehicles and vessels may display the white flag; aircraft carrying parlementaires may be painted white. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, §§ 10.4 and 10.12.
A training video produced by the UK Ministry of Defence emphasizes that the white flag or flag of truce must be respected and must not be attacked. 
United Kingdom, Ministry of Defence, Training Video: The Geneva Conventions, 1986, Report on UK Practice, 1997, Chapter 2.2.