Heatwave is not the temperature only, but in related to the regular temperature in a zone and to its season.

26°C in the desert wouldn’t be considered as a heatwave, but will be in Antarctica.

Additionally, humidity is a key factor and could affect AF. 

 Didn't think 26 C was that much for the UK in late summer 

 I guess if rain is the norm then 26c is hot 😂