Temperatures are set to hit 32 degrees in the coming days - that's hotter than it will be in Hawaii.
The main areas at risk are the south and east of England and the Midlands.
Public Health England and the NHS have issued a warning to older people, young children, and those with serious illnesses.
Christian GP Rebecca Payne told Premier: "Sun cream, although it can help, is really no substitute for being covered up.
"The other thing you can do when you're out is limit the amount of physical exertion you do. Make sure you keep drinking."
She also suggested opening windows and doors at opposite ends of the house to allow cold air to flow.
The Met Office has declared a level 2 heat-health alert. That is issued when there is at least a 60% chance temperatures will be very high on at least two consecutive days and into the night.
Forecasters say next week will see lower temperatures although it will still be very warm.
Listen to Christian GP Rebecca Payne speaking on Premier's News Hour: