Wednesday was the hottest day of the year so far, the Met Office said, with a high of 30.3C recorded at Heathrow Airport.
The previous hottest day of the year was Tuesday, which saw a high of 30C recorded in Chertsey in Surrey.
It comes as the NHS said visits to its online heat exhaustion advice page have soared in the past 48 hours.
Met Office meteorologist Kathryn Chalk said the last time 30c was recorded in the UK was on September 10 last year.
She said the weather would remain warm across much of the UK on Thursday.
“Tomorrow we’ve got a bit of patchy light rain moving eastward across England and Wales,” she said
“It’s still going to be warm ahead of that though, especially across the South East, so temperatures tomorrow probably could peak at around 27 or 28C.
“Further towards the north, actually it’s going to be feeling cool.”
The Met Office warned that it will be “unseasonably windy” in the northern half of the UK on Thursday, with gales affecting coasts and hills of Scotland and Northern Ireland.
Inland gusts of 30 to 35mph are also forecast in northern England and north Wales.
Ms Chalk said the weather would begin to become wetter and cooler as we head through Thursday and into Friday.
“We’ve got a low pressure system coming into the North West,” she said.
“That’s going to bring windy conditions and some rain and showers – so make the most of the heat today and tomorrow.”
“As we go into Friday we will see bright and breezy conditions in the north, while showers are going to be focused across Scotland.”
She said the weather would remain relatively warm with highs of around 24-25C through the weekend.
Ms Chalk added that the northwestern UK is expected to see some rain on Saturday, with much of the UK experiencing cloud and patchy rain on Sunday.
The NHS said the number of people seeking heat advice on its website has “skyrocketed” since a yellow heat health alert covering most of the country came into force on Monday.
Figures from NHS England, which runs the NHS website, show that on June 24 and 25 there were 28,116 visits to the heat exhaustion page, compared with 13,598 over June 22 and June 23 – a 107% increase.
