Lightning was also spotted in the north-east of England, with a storm breaking out near Antony Gormley's Angel of the North statue.
Hot weather may have been a "contributing factor" to a large fire which consumed about 30 acres of Thetford Forest.
Motorists on the M1 in Derbyshire faced delays after a lorry carrying batteries burst into flames, causing the motorway to be temporarily closed in both directions.
Meanwhile, five people were taken to hospital from the Royal Norfolk Show as temperatures rose to 31C (88F) in Norwich.
In Staffordshire, 80 people were stranded on the monorail at Alton Towers when two trains broke down.
Wimbledon spectators - who had been covering their heads with umbrellas, newspapers and towels - were advised to wear hats.
London was hotter than Rome and Athens, according to figures collated by the Met Office, but other parts of Europe saw temperatures rise above 40C (104F).
As hot as it gets
38.5C (101.3F)
Hottest temperature ever recorded in the UK - Faversham, Kent in August 2003
- 36.7C (98F) Recorded at Heathrow between 15:00 and 15:30 BST on Wednesday
- 7 degrees This much above average counts as "very hot" - 6 degrees over is just "hot"
- 5 days Length of time the max daily temperature must exceed average to count as a heatwave
Network Rail instructed train companies to slow down at vulnerable locations wheretracks could buckle because of the heat.
- Affected routes include Abellio Greater Anglia trains between London Liverpool Street and Ipswich and First Great Western services between London Paddington to Bourne End and Henley-on-Thames
- Virgin's East Coast line between Leeds and London Kings Cross is also running a reduced service
- Commuters have been advised to check their journeys before travelling and to carry water and sunscreen
Network Rail has said the heat could have been a factor in the derailment of a freight train in Lincolnshire on Tuesday.
Keeping cool in the heat
