As a tradition, the commercial British "lemonade" -
carbonated water,
sugar, and a hint of
citric acid - dates back only to that particular nadir of British cuisine which lasted from somewhere around the war to the mid-1970s, a period when the tendency to boil everyting to death overlapped with wartine and post-war shortages and the industrialisation of food production encouraged by successive governments' single-minded
cheap food policies.
Real lemonade, with actual lemons in it, did survive, but only for those whose mothers made it (
un nation de boutiquiers, maybe, but the American kid's lemonade stall is indeed unknown over here).
Fortunately, the relentless ferreting out of the past in the interest of being able to introduce more value added, pseudo-authentic Deep England-style lines onto the ever-growing length of supermarket shelves in the shopping cathedrals of the fairly united kingdom has led to reasonably palatable versions of real lemonade, both still and fizzy, being available once more. Just avoid the tranparent stuff.