On Saturday, Gogerddan, near Aberystwyth, recorded 27.4C – a new record for the highest late August holiday weekend temperature in Wales.
The mercury rose to 30.7C at Heathrow, west London, just shy of the record of 31.5C set in the same location in 2001.
Aberdeenshire in Scotland could see 29C on Sunday, while Northern Ireland is likely to enjoy 24C.
People attending Notting Hill Carnival on Sunday and Monday – expected to number one million – have been advised to stay hydrated.