The first world war barely registers on the graph. The second world war shows a small decrease in population. But the largest decrease is seen in the 70s and 80s. I have no idea why. But see the data for yourself:
England was barely affected by the first World War. France, Germany, Austria, Serbia, the Ottoman Empire, and Russia bore the brunt of the fighting.
And compared to the other belligerents, it was barely affected by the second, too. British colonial policies during the war killed more of their own subjects in India, then the Germans ever did on the home isles. (For anyone wondering why India sought independence... Well, that was a factor.)
The British economy nosedived into total exhaustion of resources, with resource rationing extending to 1956. It was closely intertwined with the French economy for a century before WW2 and its assets were wholly seized/decimated.
If WW2 went on for much longer then the UK would have descended into universal abject poverty and total societal collapse.
Granted this universal suffering of the populace ushered in the social-welfare state Britain is so commonly known for nowadays, it was nonetheless devastating.
That total exhaustion of resources happened due to decolonization, and taking on much debt (Much of which was inflated away over the next decade.) As it turned out, the resource shortages didn't matter, either, due to free trade. 1945 to 1960 had the highest rate of economic growth that the UK has ever experienced.
