We-ell....
In Africa, Abyssinia. Cape to Cairo Railway actually is built. ('Rah, 'Rah, Cecil Rhodes!)
In Europe, if Britain had killed Hitler, Goering and Himmler at the right time, we could have seen Germany surrender to Britain and become the Dominion of Germany.
In Asia, Persia and Iraq very nearly became British dependencies in the 1930s and 1940s. The same with Siam, for its King was afraid of a French takeover. Britain SHOULD have secured the New Territories permanently for Hong Kong.
In the 1870s, Britain handed strategic Heligoland back to Germany (hands up the number of people who really know Britain held Heligoland - and that there was a colonial flag.)
If we had really put our foot down with the Argentinians, as late as the 1920s we might have seized the place. A lot of British investment went into the development of the meat packing industry and the railways. If we and the Chileans had grabbed Tierra del Fuego or part of Patagoonia - goodbye Malvinas claims and let's drill for oil...
More regrets over Antarctica (at one time we claimed roughly 40% of it).
We should have held the Suez Canal and the Sinai to stop squabbling in the Near East. Allenby only needed a little more support to have grabbed the whole Levant up to the Turkish border.
Instead, in 1953, we took possession of Rockall.