Today is the hottest September day in over a century – with a temperature of 34.4C recorded in Kent.
The high was reached on Tuesday afternoon, according to the Met office, making it the warmest day of the year so far.
The last time it was so hot in September was in 1911.
A north-south divide was widely expected, with thunderstorms hitting the North while the South basks in hot summery sunshine.
Health warnings were issued in southern parts, but yellow warnings of rain are now in place for northern England and south-east Scotland.
The Met Office said: "A very warm and humid air mass is in place across much of the UK, with conditions marginal for thunderstorm development this afternoon as temperatures rise.
"Northern England and southeast Scotland are currently judged the most likely area to see scattered thundery downpours, but even here many places will have no more than light rain.
"However, where storms do form, there is a lot of moisture and energy available, with the risk of more than 30mm of rain in around an hour, large hail and frequent lightning."
The yellow warnings are valid from 2pm to 11pm.
Joe Mulligan, head of first aid education at the British Red Cross, said: "Most heat disorders occur because someone has been over-exposed to heat or has over-exercised for his or her age and physical condition.
"Simple steps such as avoiding exposure to the hottest time of the day, drinking plenty of fluids and even simply wearing a hat on hot days can all make a real difference."
