My best build yet! Look at this Glorious P51 in the colors of His Majesty’s Royal Air Force
The Mustang was initially developed for the RAF, which was its first user. As the first Mustangs were built to British requirements, these aircraft used factory numbers and were not P-51s; the order comprised 320 NA-73s, followed by 300 NA-83s, all of which were designated North American Mustang Mark I by the RAF.[28] The first RAF Mustangs supplied under Lend-Lease were 93 P-51s, designated Mk Ia, followed by 50 P-51As used as Mustang Mk IIs.[29] Aircraft supplied to Britain under Lend-Lease were required for accounting purposes to be on the USAAC's books before they could be supplied to Britain. Lend-Lease aircraft destined for the RAF were first ordered and paid for on Britain's behalf by the USAAC.[citation needed]

After the arrival of the initial aircraft in the UK in October 1941, the first Mustang Mk Is entered service in January 1942, the first unit being 26 Squadron RAF.[30] Due to poor high-altitude performance, the Mustangs were used by Army Co-operation Command, rather than Fighter Command, and were used for tactical reconnaissance and ground-attack duties. On 10 May 1942, Mustangs first flew over France, near Berck-sur-Mer.[31] On 27 July 1942, 16 RAF Mustangs undertook their first long-range reconnaissance mission over Germany. During the amphibious Dieppe Raid on the French coast (19 August 1942), four British and Canadian Mustang squadrons, including 26 Squadron, saw action covering the assault on the ground. By 1943–1944, British Mustangs were used extensively to seek out V-1 flying bomb sites. The last RAF Mustang Mk I and Mustang Mk II aircraft were struck off charge in 1945.

Info:

A Royal Air Force North American Mustang Mk III (FX908) on the ground at Hucknall
The RAF also operated 308 P-51Bs and 636 P-51Cs,[32] which were known in RAF service as Mustang Mk IIIs; the first units converted to the type in late 1943 and early 1944. Mustang Mk III units were operational until the end of World War II, though many units had already converted to the Mustang Mk IV (P-51D) and Mk IVa (P-51K) (828 in total, comprising 282 Mk IV and 600 Mk IVa).[33] As all except the earliest aircraft were obtained under Lend-Lease, all Mustang aircraft still on RAF charge at the end of the war were either returned to the USAAF "on paper" or retained by the RAF for scrapping. The last RAF Mustangs were retired from service in 1947.[34]

Source: Wikipedia which is totally correct.