-Britain keeps the 13 colonies
-British North America still expands all the way to the Pacific, reaching a size a little larger than the OTL US+Canada combined
-Britain takes Indonesia from the Dutch in the early 19th century
-A different version of the Scramble for Africa happens, with Britain and France taking the lion's share.
-Some kind of massive global war goes down in the 20th century (Britain and somebody else vs. France and Russia). Russia is completely obliterated, and Britain takes all of Siberia. Britain takes all of France's colonies too, then goes ahead and claims all of Antarctica and the Arctic.
Final size of the British Empire: OTL+nearly all of Africa+South and Southeast Asia+USA+Siberia+Antarctica+the Arctic.
That has to be pretty close to 1/2 of the world's territory.