It's the RAC Flash. Introduced in 1917 British tanks were given a prominent white/red/white mark to easily distinguishable from captured units used by the emeny, the colours being those of the Cavalry Corps.
The red/white/red AFV recognition sign was introduced in March 1942 consisting of a rectangle 10-in high and divided vertically into 6-in red/white/red strips. It was to be carried on all AFVs in the UK and with the 1st Army in Tunisia, later on it was used by all British armoured units in Italy. It offically remained in use until the spring of 1944. In the North Africa it was intended to be white/red/white but it was soon abandoned as it provided a good aiming mark for enemy gunners.
British Military Markings 1939-1945, Hodges & Taylor, and
British Tanks Markings and Names, White