For London and the South East to record a heatwave, temperatures must exceed 28C for at least three days, while this drops to 27C in the Midlands and 26C in the South West.
In Northern Ireland, Scotland, Wales, Devon and Cornwall and the North East it is 25C.
Warnings about the extreme heat have been issued by both medical authorities and vets, urging people to keep the vulnerable and animals safe.
PHE advised people to look out for those who may struggle to keep cool and hydrated, such as older people and those who live alone.
Emergency animal care provider Vets Now also warned rising temperatures could increase the risk of heat stroke in dogs.
The vet service sees a spike in callouts as the weather warms up and has warned that temperatures above 20C put dogs at risk, and that survival rates for dogs with heat stroke is just 50 per cent.
The warnings come after a week of flash flooding in the south east of England, causing disruption to transport in London.