The Met Office said the highest temperatures are expected in London and the South East, with 35C (95F) expected on Thursday and 37C on Friday.
The all-time UK record is 38.5C (101.3F), set in August 2003, and the highest July temperatures was 36.7C (98.06F) in 2015 at Heathrow.
Wednesday’s top temperature was 32C (89.6F) in Wisley, Surrey.
The past few weeks have seen the driest first half of summer on record, leaving grasslands parched and parts of the country facing water restrictions.
Some train operators have introduced speed restrictions, crops are being damaged, and people have been warned to stay out of the sun between 11am and 3pm.
In East Anglia and the south-east of England, the last day of widespread rainfall was 29 May.
Paul Gundersen, chief meteorologist at the Met Office, said: “If you’re looking for somewhere to escape the heat, western and northern areas will have pleasantly warm mid 20s, although across Northern Ireland and western Scotland this may be accompanied by occasionally cloudy skies.”
But thunderstorms could be on the way, with possible flooding, difficult driving conditions and power cuts.
The Met Office has issued a yellow warning for torrential downpours, hail and strong wind on Friday afternoon and evening.
This warning covers the south-east of England, along with northern and eastern parts of the country.
Mr Gundersen added: “There is the chance of thunderstorms breaking out over some eastern parts of England on Thursday, but it is Friday when we see intense thunderstorms affecting many central and eastern areas.
“Whilst many places will remain dry and hot, the thunderstorms on Friday could lead to torrential downpours in places with as much as 30mm of rainfall in an hour and 60mm in 3 hours.
“Large hail and strong, gusty winds are also likely and combined could lead to difficult driving conditions as a result of spray and sudden flooding.”
Meanwhile, Sky Data interviewed 1,243 customers to find out what they thought about hosepipe bans and whether climate change was to blame for the heatwave.
Some 32% said they thought climate change was a great deal to blame for the recent hot weather, while 10% said it was not at all.
Also, 12% said using a hosepipe during a hosepipe ban was socially acceptable and 68% said they would not find a hosepipe ban disruptive.
Sky Data interviewed a nationally representative sample of 1,243 Sky customers by SMS on 25 July 2018. Data are weighted to the profile of the population.
For full Sky Data tables, please click here.