British colonized half of the world. But access to clean water was in some parts of the world problematic and if a foreigner unaccustomed to local water wanted to avoid problems with a bowel, the water had to be boiled. However, boiled water can stink incredibly and taste even worse. Hence the water had to be flavored either by tea or coffee. And it happen that British colonized the part of the world which was suitable for growing of tea and not coffee.
But why British drink unsweetened tea? French food is best with wine. German Schweinebrate is best with beer. And incredibly greasy Russia food is best with vodka. Drinks complement food. And unsweetened tea complements British food. Hence I will begin from food. British food is traditionally either salty, acidic or sweet. It is because at the beginning of the medieval time the average temperature decreased by two Centigrade and British islands began to be dependent on imported of food. But what methods could be used to preserve the food during the transport in the medieval times? Basically four methods were possible: put it into salt, put it into honey, put it into vinegar or smoke it.
But British never used smoking very much since smoking is good only for preservation of meat, fish or cheese. But British are on an island and have the finest grass in the world that has been extensively used for breeding of domestic animals. Hence British never really needed to import any meat, fish or cheese and never needed to eat anything smoked. And after almost complete eradication of all British forests during the Victorian epoch for ships they even couldn't enjoy domestic smoked products.
Consequently all imported food must have been salty, acidic or sweet. But try to eat salty or acidic things all the time. I will spare you and your stomach - it is impossible. Eat too much salt and you get into a problem with osmotic pressure. Eat too much acidic food and you run into a problem with pH. Hence most British food was sweet - it was the only way how to preserve the food while preserving its edibility. And traditions are extremely strong in England. Thus British still eat over-sweetened food, they still drink tea and they still don't put any sugar into tea because all the sugar is included in the food. British are so keen to make everything sweet that even meat is caramelized to make it taste sweet. And the only way how to survive the never ending flood of sweet things is to drink bitter tea. And reversely, the only way how to survive bitter tea is to eat sweet food. Hence it is Ouroboros.