The Met Office says temperatures are likely to climb above 20 celsius.
There were no reports of any injuries after the London Midland service from Milton Keynes to London Euston came off the track at 7am.
Pictures on social media show severely waterlogged rail stations and roads as a weather warning was issued.
It was also the warmest temperature recorded in September in Britain since 1911.
Temperatures reached 32.2°C at Heathrow by lunchtime.
Expect a little more cloud in this afternoon, particularly in the west although it will still remain dry with plenty of sunshine.
Expect rain clearing slowly and erratically east through this afternoon and evening before a much brighter start with sunny spells tomorrow.
Expect some sunny spells with odd spots of rain for Sunday morning with scattered, heavy and possibly thundery showers in the afternoon.
The Met Office says temperatures are expected to be highest in East Anglia and down to London.