This spectacular slow-motion footage shows a lightning strike over Cheltenham in Gloucestershire.
An amber weather warning is in place across much of England and Wales for heavy rain and thundery downpours.
There may be some drier and brighter weather around this afternoon - more so for the north-east of England - but elsewhere it stays very changeable.
Temperatures are still on the hot side down in the south-east, elsewhere it will still be in the low twenties.
It will generally be an unsettled day across the British Isles today. Areas of heavy showers and thunderstorms, which will be severe in places, will move northwards across many parts.
Some drier and brighter spells are also expected and it will continue to feel humid.
Temperatures becoming very warm, locally hot in the east, with highs of 27C (81F).
One in five flights will be cancelled during a two-hour period at Heathrow today due to thundery weather, the airport has said.
A spokeswoman said: "There is severe weather forecast at Heathrow with a 70% risk of thunderstorms, lightning strikes and hail storms. During thunderstorms the number of aircraft that can take off and land each hour is reduced.
"We have therefore requested that airlines reduce their flight schedule by 20% between 4pm and 6pm. Passengers due to travel on July 19 are urged to check the status of their flight with their airline before travelling to the airport."
Despite expected storms, forecasters are also warning that Britain's heatwave could continue in parts of the country until tomorrow morning.
A "yellow" warning of a heatwave has been issued for the UK today, advising people to be "alert and ready".
The Met Office said there was a 60% chance of a heatwave continuing until early Sunday, with high levels of humidity.
But in an "amber" warning of rain for England, eastern and central Wales, the organisation also warns that heavy, thundery showers that developed overnight will move northwards during the day.
The Met Office said: "Not everywhere within the warning area will see thunderstorms, and indeed some spells of warm sunshine are expected at times, but where thunderstorms do form, some torrential downpours are likely with frequent lightning, large hail and locally strong gusts."
After basking in the year's hottest temperatures at the end of the week, Britain is now braced for torrential downpours and storms that could cause flash flooding across large parts of the country.
The Environment Agency warned that up to 30mm (1.2in) of rain could fall in an hour in some areas.
The agency says that almost every county in England and Wales is at "medium" risk of flooding today, and has issued 59 flood alerts at the time of writing, warning people to be prepared.
The majority of these are in the Midlands, the South East and the South West.
Lightning strikes continued across the UK on Friday night, just hours after Brits enjoyed temperatures of 32C.
The conditions seemed to keep many people awake, after a #thunderandlightning began trending on Twitter in the early hours of Saturday.
More than 3,000 strikes were reported across the UK in just two hours on Friday morning.
Police in north London were left with no choice but to smash the windows of this car when they found a dog trying to shelter on the pedals.
The owners of the car near Finsbury Park returned to find the back window shattered.
Over 3,000 lightning strikes were reported across the UK in just two hours this morning. The storms were caused by hot air pushing up from the south.