The previous high was 28.3C in Northolt, north-west London, on June 2.
Temperatures could hit 30C by Sunday afternoon, which would make it the warmest June 13 on record.
Sunday is predicted to be a ‘fine, dry day for most’ with highs of 29C in London, 23C in Edinburgh and 24C in Cardiff.
Simon Mackie, 54, was putting his recycling bins out when he looked up to see the rare cloud formation on Sunday evening.
Dry and sunny conditions are expected to return later in the week.
Temperatures may rise on Sunday, but showers and thunder are forecast for parts of England.
Fine weather is expected in many parts of the country but rain and thunderstorms are likely in south-west England and Wales.
A balmy 25.7C was recorded at Heathrow Airport at 2pm on Tuesday, making it the hottest day of 2021 so far.
Torrential downpours have led to the UK recording more than its average rainfall for May but forecasters say it will be drier and warmer in the coming days.
The 60-mile race was being held in the Yellow River Stone Forest tourist site.
Parts of the country could see weather warnings imposed with heavy downpours and gales set to move in, the Met Office has said.
The Met Office long-range forecast for May 17-26 warns it will ‘most likely be remaining unsettled throughout’.
Filmed at midnight in Guiyang, the video shows red luminescence clouds shining and flickering in the night sky.
It came as ‘unusually’ cold air for May moved south across the country, dragging the temperatures down to between 2C and 4C.
A yellow weather warning for wind is in place until 9am on Tuesday.
Weather warnings could be imposed over the weekend, the Met Office has said.
Britain is enjoying a sustained period of April sunshine but one region of the UK will be particularly pleasant this weekend.
‘We’ve had fabulous sunshine, then snow, then sunshine again. And now? Both together!’ tweeted one onlooker.
Highs of nearly 18C were recorded on Sunday, but will fall into single digits for Monday.
Met Office forecaster Alex Burkill said it was a ‘possibility’ that the March record could be broken on Wednesday.