Thunder storms swept across much of the UK on Tuesday night, ahead of possible record-breaking heat this week.
Houses in Nuneaton, Warwickshire, and Wrexham caught fire after being hit by lightning, while a taxi driver had to be rescued from flood water in Newbridge, Caerphilly county.
Elsewhere, police are searching the River Thames in London following reports of people going into the water.
BBC Weather said there were about 48,000 lightning strikes overnight.
At least 20-30mm of rain fell in parts of Wales, the Midlands and the north of England.
Forecasters say temperatures in some parts could reach 36C (97F) later.
Met Office meteorologist Emma Smith said temperatures in some places had climbed back up to 23C or 24C by 05:00 BST.
“It’s normally 13 or 14 degrees at this time of year, so that’s 10 degrees above average,” she added.
Police divers were called out on Tuesday evening to three different stretches of the Thames in Greater London, following reports of people getting into difficulty in the water.
The Metropolitan Police said on Wednesday that river searches were continuing at Shadwell, Waterloo, and Kingston.
On Thursday, temperatures in eastern England could surpass the current July record of 36.7C set at Heathrow in 2015, and possibly the all-time record of 38.5C set in Faversham, Kent, in August 2003.
Northern Ireland and western Scotland are expected to be the coolest areas – with temperatures in the low 20s.
Rail operator Southeastern has said it will run a “significantly reduced service” on Thursday due to speed restrictions announced by Network Rail as a result of the forecast high temperatures.
Precautions are put in place as rails can buckle in the heat.
Southeastern operates trains in south-east London and Kent and also serves parts of East Sussex.
Meanwhile, Western Europe is braced for temperatures of more than 40C.
The French city of Bordeaux hit its highest temperature since records began on Tuesday – with Meteo France registering 41.2C (106.1F).
Forecasters have also predicted a record-breaking temperatures in other countries including Belgium, Germany and the Netherlands.
A World Meteorological Organization spokeswoman said the heatwaves bore “the hallmark of climate change”.