The UK is bracing for its 'hottest day ever' with nearly 500 schools set to close early, army ceremonies cancelled and people warned not to travel as a heat dome spreading across Europe reaches the country.A red weather warning has been issued for tomorrow and Thursday, with temperatures due to come close to Britain's all-time record of 40.3C, set in July 2022. Some schools have cancelled trips and are setting work for students to do at home, after the Met Office warned the heatwave is expected to bring 'severe and significant impacts', including widespread health risks for many. Meanwhile the Army has cancelled ceremonial operations in London and Windsor to protect the 'wellbeing' of its soldiers, and lidos are already fully booked for the week ahead. Extreme heat is also expected to cause travel disruption tomorrow with train services across Kent and south-east London likely to be affected, South Eastern Railway said. The rail operator urged passengers to only travel if 'absolutely necessary'.The highest temperature recorded in the UK on Tuesday was 34.6C in Wisley in Surrey, the Met Office has said.Scotland and Northern Ireland recorded their hottest days of the year so far with the mercury hitting 28.8C in Fyvie Castle, Aberdeenshire, and 26.4C in Katesbridge, County Down. It comes after a violent band of storms swept across southern England overnight, forcing at least one airport to close, rail services to be thrown into chaos, and around 3,000 lightning strikes to hit London in just two hours. Two homes were set on fire in the capital, reportedly as a result of lightning strikes, with the London Fire Brigade receiving more than 400 calls since midnight.