Thunder and heavy downpours struck from the south coast to the north of England overnight as the weather front swept north into Scotland.
Forecasters had predicted another stormy night before a return to above average temperatures over the weekend, punctuated by more patchy storms.
The Met Office has issued a yellow "be aware" severe weather warning for England and southern Scotland, which runs to noon today, and warned of possible "localised surface water flooding" caused by torrential rain.
It follows a week which saw the hottest July day on record and similar storms featuring golf ball-sized hail which caused an estimated £1.5 million in damage to cars.
The mercury could rise to 29C (84F) in East Anglia later while temperatures are set to be above average elsewhere.
John Griffiths, a forecaster for MeteoGroup, said: "Thundery showers covered from the north of England down to London and there have been quite a few lightning strikes in London and the south coast, as well as in Wales.
"It will continue to push northwards into Scotland and the north of England, which will have heavy rain early on Saturday but they should clear by evening. In the South it will be mainly dry.
The Thunder and lightning last night was actually amazing...I wish it could thunder and lightening every night. pic.twitter.com/lcfvX7t2gU— Aiden Thomas taylor (@aidenthomas221) July 4, 2015
"The front came in an arc through Ireland to London via the north of England.
"It's going to stay pretty showery on Sunday with sunny spells and thundery outbreaks most likely in the South West and most of the west coast."
Fork and sheet lightning were seen in areas including London, Bournemouth, Southampton and Surrey as social media buzzed with people sharing their lightning pictures.
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The fresh storms come as some householders and businesses in the north of England and Scotland are counting the cost of the wave of freak weather that followed Wednesday's record-breaking temperatures.
AA Insurance said it took dozens of claims for cars wrecked by hailstones, and estimated around 1,700 cars worth up to £1.5 million could have suffered.
And the Met Office said the 34-hour period to 10am Thursday saw 19,525 lightning strikes, 15,273 of which were in Scotland.
On Wednesday, the highest temperature recorded was 36.7C (98.1F) at Heathrow, breaking the record for a July day, while many other places broke the 30C (86F) mark.