Aircon was not around 40 years ago in Greece etc in a lot of holiday accommodation. I holidayed there many times and it was 35C without any aircon in villas.
Also, the 40C is a prediction. It doesn't apply to all of the UK. It may not even reach 40C.
The warnings are, IMO, there to prevent illness and accidents that most often occur when people do not take care. (Swimming in rivers, jumping off cliffs, lying on the beach, not maintaining cars so they break down....)
Most people I know who can, are WFH, or driving to work in a car with aircon, into an office with aircon, and some have an aircon unit in their own homes.
I feel sorry for others who can't do this and hope their employers will be considerate.