The record has been broken for the highest temperature recorded in October - now at 29.9C (85.8F).
It was set at 14:42 BST in Gravesend, Kent, beating the previous record of 29.4C (84.9F) recorded on 1 October 1985, in March, Cambridgeshire.
In Wales, a new October record was set at 28.2C (82.7F) in Hawarden, Flintshire, at 14:12 BST, the Met Office confirmed.
Temperatures in England topped those in Athens, Los Angeles and Barcelona.
But it was a different story in parts of Scotland and Northern Ireland with outbreaks of rain and temperatures tipping just 17C (62.6F).
In Edinburgh on Wednesday the high was 24.7C (76.4F) - the warmest day in Scotland for the time of year for more than 50 years.
Met Office forecaster Andrew Sibley said: "We have had southerly wind for several days which has brought very warm air up from the south."
The record was broken in Gravesend at 13:27 BST when temperatures soared to 29.5C (85.1F)
Lauren Cherry, manageress of The Rum Puncheon Public House, Gravesend, said it had been a busy day so far.
"We've been rushed off our feet because we have a patio overlooking the Thames.
"Everyone's been quite shocked about the weather but pleased. I'm surprised we broke the record."
But fellow publican Paul Drake, landlord of the Jolly Drayman, said it had been a quiet afternoon - blaming the economy.
"Everyone's at home cooking on their barbecues and drinking their beer from Tesco," he added.
Meanwhile thousands of people headed to beaches across the south of England.
In Brighton alone, Visit Brighton predicted that 300,000 sunseekers would flock to the seafront over the weekend.
Officials reported a 30% increase in people clicking on to the city's official tourist website and calls to its visitor information centre rose by more than half.
Tourism councillor Geoffrey Bowden called the visitor boost a "welcome fillip" at the end of the summer season, which helped to support the 13,500 jobs dependent on tourism there.
BBC Weather forecaster Holly Green said it would be another hot day on Sunday and temperature records could be challenged again.
"It's going to be another hot day in store, mainly across the south-eastern parts of the UK," she said.
"We are losing heat by Tuesday so temperatures will become much closer to what you would expect at this time of the year. Things are becoming cloudier and breezier.
"The unsettled picture for northern parts of the UK will stay that way for next week."
RNLI lifeguards will patrol 12 beaches across Devon and Cornwall on weekends in October and the October school half term.
Fistral beach, Newquay, will be patrolled during the whole of the month, as good surf conditions attract hundreds of surfers to the water.
Meanwhile, experts at the Royal Horticulture Society (RHS) said confused plants started to flower again due to the unseasonably warm weather.
Strawberries and rhododendrons were among the plants seen blooming at its flagship garden in Surrey when they were not expected to flower again until next spring.