A level 3 weather alert has recently been declared in many parts of the UK due to the very hot weather.
This is declared when daytime temperatures have exceeded, or are reliably forcast to exceed about 30 degrees for at least 2 consecutive days, with nightime temperatures not falling below about 17 degrees during the intervening night. (the exact temperatures vary slightly by region)
The level 3 alert is basicly a warning that actual action is required rather than just pre planning.
Level 4, which is a national emergency has not been declared and probably wont be.

The present weather is nothing remarkable for some places, but is unusually hot by UK standards.
Most UK homes are not air conditioned, and traditionly this has not been needed.
Most large offices and department stores are air conditioned, with very varying degrees of reliability and effectivnes.
Most smaller busines premises are not air conditioned.