24-26" is not that extreme. I routinely see original surplus trousers about that range. Our modern Western Diets and habits don't contribute to such slimness these days. Remember that Britain on the whole was not as properly nourished as they should have been prior to the War. Lord Woolton contributed greatly to correcting the national diet and equalizing availability of quality food.
If you are interested, here is a link to a video series done on the subject of British Rationing during the war. https://www.youtube.com/playlist?lis...k_n9ebj12102sb