Question: Since when he Francis Wallace Grenfell's rank in the British Army been the highest rank?

Context paragraphs:
General (Sri Lanka): General is the second highest rank in the Sri Lankan Army and is a four-star rank.  The rank is held by a Chief of the Defence Staff or is mostly awarded as a ceremonial rank to retiring Commanders of the Army.  The serving Commander of the Army since the 1970s have head the rank of Lieutenant-General with a brief exception in 2009 when Field Marshal Sarath Fonseka was promoted as a General while he was still the Army Commander.  Sepala Attygalle was the first to be appointed to the rank of General.  Three former/retired Volunteer Force officers have been appointed to the rank, these include Sir John Kotelawala, Ranjan Wijeratne (posthumously) and Anuruddha Ratwatte.

List of British Army full generals: This is a list of full generals in the British Army since the Acts of Union 1707.  The rank of general (or "full general" to distinguish it from the lower general officer ranks) is the highest rank currently achievable by serving officers in the British Army.  It ranks above lieutenant-general and below field marshal which is now only awarded as an honorary rank.  The annotation "Held rank in the East Indies."  indicates that the officer served in India in the East India Company's army.

General (United Kingdom): General (or full general to distinguish it from the lower general officer ranks) is the highest rank currently achievable by serving officers of the British Army.  The rank can also be held by Royal Marines officers in tri-service posts, for example, General Sir Gordon Messenger the new Vice-Chief of the Defence Staff.  It ranks above lieutenant-general and, in the Army, is subordinate to the rank of field marshal, which is now only awarded as an honorary rank.  The rank of general has a NATO-code of OF-9, and is a four-star rank.  It is equivalent to a full admiral in the Royal Navy or an air chief marshal in the Royal Air Force.

Commissioner (The Salvation Army): The rank of Commissioner in The Salvation Army is the second highest rank attainable by Officers in the organisation, and many of the Army's Territorial Commanders and even the Chief of the Staff hold this rank (the highest rank in The Salvation Army, the rank of General, is by election).  The rank of Commissioner has been an active rank since 1880, and is one of the original ranks created by General William Booth, the first appointed Commissioner being George Scott Railton.

Lieutenant general (Bangladesh): Lieutenant general (abbreviated as Lt. Gen. and in contraction simply known as 'general') is a senior rank in the Bangladesh Army.  It is the second-highest active rank of the Bangladesh Army and was created as a direct equivalent of the British military rank of lieutenant general.  It is also considered a 3 star rank.  Till 2007 it was the highest rank in the Bangladesh Armed Forces as all chiefs of army staff held this rank and navy and air chiefs were of two-star ranks.

Marshal of the Royal Air Force: Marshal of the Royal Air Force (MRAF) is the highest rank in the British Royal Air Force (RAF).  In peacetime it was granted to RAF officers in the appointment of Chief of the Defence Staff (CDS), and to retired Chiefs of the Air Staff (CAS), who were promoted to it on their last day of service.  While surviving marshals of the RAF retain the rank for life, the highest rank to which officers on active service are promoted is now air chief marshal.  Although general promotions to Marshal of the Royal Air Force have been discontinued since the British defence cuts of the 1990s, further promotions to the rank may still be made in wartime, for members of the Royal Family and certain very senior RAF air officers in peacetime at the discretion of the monarch; all such promotions in peacetime are only honorary, however.  In 2012, Charles, Prince of Wales was promoted to the rank while in 2014 Lord Stirrup, who had served as Chief of the Air Staff and Chief of the Defence Staff for over seven years, was also promoted.

General (Yugoslav People's Army): General (YPA) or General (J.N.A.) (Serbo-Croatian and Slovenian: "General (Jugoslovenska Narodna Armija)", Serbian Cyrillic: "Генерал (Југословенскa Народнa Армијa)") was the highest rank of Yugoslav People's Army.  (The highest rank in theory, Marshal of Yugoslavia, was created for Josip Broz Tito and held by him alone).  The only person to ever considered for promotion in rank of "General" (General (YPA)) was Ivan Gošnjak as deputy supreme commander.  This rank was created in 1955 for deputy supreme commander of armored forces SFRY and it was abolished in 1974.  After that the highest rank of Yugoslav People's Army was General of the Army for Yugoslav Ground Forces and Yugoslav Air Force, and Admiral of the Fleet for Yugoslav Navy.

Field marshal (United Kingdom): Field Marshal has been the highest rank in the British Army since 1736.  A five-star rank with NATO code OF-10, it is equivalent to an Admiral of the Fleet in the Royal Navy or a Marshal of the Royal Air Force in the Royal Air Force (RAF).  A Field Marshal's insignia consists of two crossed batons surrounded by yellow leaves below St Edward's Crown.  Like Marshals of the RAF and Admirals of the Fleet, Field Marshals traditionally remain officers for life, though on half-pay when not in an appointment.  The rank has been used sporadically throughout its history and was vacant during parts of the 18th and 19th centuries (when all former holders of the rank were deceased).  After the Second World War, it became standard practice to appoint the Chief of the Imperial General Staff (later renamed Chief of the General Staff) to the rank on his last day in the post.  Army officers occupying the post of Chief of the Defence Staff, the professional head of all the British Armed Forces, were usually promoted to the rank upon their appointment.

Ranks of the Bangladesh Army: The Bangladesh Army (Bengali: বাংলাদেশ সেনাবাহিনী ) is the land forces branch of the Military of Bangladesh.  The rank structure of the Bangladesh Army is divided in two main parts: commissioned officers/officers and soldiers, officers have two categories of ranks one is known simply as officers and the other category is generals or general officers, on the other hand soldiers have three categories of ranks.  Junior Commissioned Officers (JCOs) and NCOs are enlisted personnel, not officers.  From the rank of Second Lieutenant to the highest rank of General all are commissioned officers.  Commissioned Officers are regarded as 1st class Bangladesh-government officer.  Men and women can receive commission from BMA, Bhatiary.  'Sainik' (Bengali: সৈনিক ) is a Bengali word, it means Soldier in English, the rank "Sainik" is equivalent to 'Sepoy' or 'Jawan' of Pakistan Army and Indian Army.  From a "Sainik" a person is promoted to Lance Corporal, and from this rank he/she gets NCO (Non Commissioned Officer) status.  From the rank of 'Warrant Officer' persons are called JCO.  From Major-General to General, officers of these rank holders are commonly called 'generals' and Brigadier-generals are known as Brigadiers in contraction, the suffix 'general' was added in 2001.  2nd Lieutenant, Lieutenants and Captains are considered junior officers (not junior commissioned officer).  Major and Lieutenant-colonels (commonly called as colonels) are mid-ranking officers.  Colonel and above are considered as senior officers.  The insignias of Major-general and Lieutenant-general and full-general were changed in 2013, previously Major-General insignia was a pip over a crossed sword and baton and Lieutenant-General was 'shapla' (water lily) over a crossed sword and baton and full-general had a 'shapla' over a pip over crossed sword and baton.

Francis Grenfell, 1st Baron Grenfell: Field Marshal Francis Wallace Grenfell, 1st Baron Grenfell, GCB, GCMG, PC (29 April 1841 – 27 January 1925) was a British Army officer.  After serving as aide-de-camp to the Commander-in-Chief, South Africa, he fought in the 9th Xhosa War, the Anglo-Zulu War and then the Anglo-Egyptian War.  He went on to become Sirdar (Commander-in-Chief) of the Egyptian Army and commanded the forces at the Battle of Suakin in December 1888 and at the Battle of Toski in August 1889 during the Mahdist War.  After that he became Governor of Malta and then Commander-in-Chief, Ireland before retiring in 1908.

Supporting paragraphs: Francis Grenfell, 1st Baron Grenfell Field marshal (United Kingdom)
Answer: 1736