Britain won, on average, 17 gold medals per Olympics in the five official games held in its imperial heyday before World War I. That average has dropped to only five medals per Olympics in the 17 held since.
Nowadays, Britain gets an average of five gold medals per Olympics. 
mnli hypothesis: Nowadays, Britain gets an average of five gold medals per Olympics.  premise: Britain won, on average, 17 gold medals per Olympics in the five official games held in its imperial heyday before World War I. That average has dropped to only five medals per Olympics in the 17 held since.