If one looks back to older recipes such as those presented in The English Huswife by G. Markham through to The Scots Kitchen, by F. McNeill we see a breadth of ingredients and cuisines available in British history. Additionally, British baking history is brimming with historical recipes. Plum cakes spring to mind, but a great many of the most celebrated bakes are quite old indeed.
So, what happened? The short answer is rationing. The system introduced in WWII by Lord Woolton did away with a large amount of spices, sugar, and other non-essential “luxuries.” As biographied in Eggs or Anarchy by W. Sitwell, Lord Woolton had a delicate digestion that reacted poorly to rich or spiced foods. In addition to his own views, the wider strategic view that anything not strictly essential to sustenance was an unnecessary waste of shipping capacity meant that these spices disappeared quite suddenly.
Of course, the British people had to adapt to this new system. The government attempted to aid this by providing example recipes within the Rationing system. The Ministry of Food released many leaflets over the years of the war, which you can read in a collected format in Food Facts for the Kitchen Front. This, sadly, is where it all goes downhill. The famous examples of “Mock Duck” and other less favoured British foods are present. The instructions often told cooks to overboil food, and seasoning was limited in the directions. Spices were almost non-existent.
Now, we have to note some successes here. Carrot Cake was born from these leaflets, as an attempt to make sweet cake with little or no sugar, using an easily grown native vegetable. Apple Crumble also made its debut in this era, as a simple dish that required less resources than a full Apple Cake. Since foraging and growing your own fruit was permissible, it was an option for many to collect apples for this dish, making it quite inexpensive.
Despite these few successes, a diet of unspiced, overboiled, minimally seasoned food using a limited selection of ingredients had been forced, unilaterally, upon a populace. Even fine dining establishments and upper class households were beholden to it. Lord Woolton intended for the system to appear truly equal and fair to all. Even the King and Queen famously greeted Eleanor Roosevelt with slices of National Loaf for afternoon tea. The West End Front by M. Sweet is quite a good book for looking at attempts by high end hotels and restaurants to continue to offer exciting meals to their clientele.
Since the Rationing was so all encompassing, and lasted from 1939 until 1955, it left a culinary mark on a generation. That persisted through the late 20th Century as ingredients, techniques and recipes gradually returned or were introduced.