This dates back to the early 1930s, when the British government wanted to introduce a standardised multi-service uniform template, especially as the then-antiquated 'Service Dress'/No. 2 Uniform was unsuitable for mechanised infantry and was hated for being uncomfortable and difficult to modify for different environments. This would lead to the adoption of 'Battledress'/BD'/No. 5 Uniform.

As part of this modernisation effort there would be pockets spread across the uniform in carefully-placed areas so as not to interfere with fighting and to provide the exact space for the soldier's exact load. This was a revolutionary improvement over the uniforms of pre-1930, which often relied on belts of leather or serge compartments to carry equipment.

It was agreed that the trousers would have a pocket designed for a field-dressing and a pocket for maps or paperwork in addition to the four in the normal places. The large pocket you've pointed out is the 'map pocket' - although it would also be used for carrying orders, messages, and the soldier's identification papers (and, inevitably, personal letters, wills, and photographs.) These map pockets can still be seen on some modern British and international uniforms, as well as Cold War examples like yours.

This was the first time someone had designed trousers specifically for rugged durability and carrying large amounts of equipment, and would be copied by many militaries around the world. One of the best designs to follow was the American M-1942 Paratrooper Uniform, but there were many other designs that improved on the originals. As with most military fashion, these new 'cargo pants' would enter the civilian sphere; but I think that's an answer for another day!