The hottest day ever recorded in the UK was on 11 August 2003. The highest temperature recorded on that day was in Gravesend in Kent, and reached 38.1 C or 100.6 F. This is definitely hotter than the previous record, which was set in Cheltenham in 1990 at 37.1 C. On that day in August 2003 temperatures were at record levels elsewhere in the country to - for instance, at Heathrow they reached 37.9 - so it was an exceptional day in most of the country. It also soon became a very wet one, with heavy rain and storm especially in the north.
38.1 C may not sound like much compared to the temperatures reached in other parts of the world, but in the UK this was the hottest recorded since records began in 1875, and with all the humidity the temperatures were quite uncomfortable for many.
This year (2013) has yet to beat the record set 10 years ago, but of course the summer is far from over yet.