Some Background Information from the Imperial War Museum

The Royal Tank Corps (RTC) was the first to wear the beret in British service as a uniform item. Its origins date back to 1922 when the Colonel Commandant of the RTC, General Sir Hugh Elles, recommended the adoption of a headdress similar in form to that worn by French troops of the 70th Chasseurs Alpins who he had observed training with the Tank Corps in 1918. Approved in 1924 the black beret was a very practical item when worn within the close confines of the tank. The beret, over time attained a near 'elite' status, as it was subsequently adopted by other 'specialist' branches of the Service, notably airborne troops, SAS, Army and Royal Marine Commandos. Post Second World War, when the detested Cap, GS was phased out, the beret was adopted universally, khaki for most regiments, airborne and armoured troops retaining their maroon and black berets, respectively. In 1950 a beret of midnight blue was issued to all except Scottish, airborne and tank personnel and worn for all normal duties within the UK and abroad. The Guards, Royal Horse Artillery and Military Police however retained the use of the pre-war Service Dress Cap.