Looking at the met office previous 24 hour records it looks like the main contenders were Crosby, Heathrow and Benson.
Crosby kept good heat up until 6am, where it was still 22.6c, but it then fell off a cliff down to 19.1c at 7am
Benson kept its heat for longer, then dropped to 22.0c by 7am
Heathrow kept most of its heat falling to 22.8c at 5am and climbing shortly after, I'm pretty sure that this was the highest minimum temperature last night, the site bottomed out at 22.8c again this morning at 10am
The UK overall record is 23.9c from Brighton, 3rd August 1990, I have no idea of local London records
http://www.metoffice.gov.uk/public/weather/observation/map#?map=Temperature&zoom=7&lon=-2.89&lat=53.19&fcTime=1468944000
http://www.metoffice.gov.uk/public/weather/observation/gcpsvg2yz