Information on update: End time brought forward, probability of impacts increased, and emphasis switched more from thunderstorms towards heavy rainfall for most of the area. Spells of prolonged and often heavy rain will continue to affect parts of England today, before clearing from most areas this evening.
20-40 mm of rain will fall quite widely, with a few locations seeing 50-70 mm within around 12 hours - the wettest weather is likely to be over parts of East Anglia, the East Midlands and southeast England, as far west as Hampshire.
Some thunder and lightning is also possible, most likely over parts of East Anglia and southeast England but heavy rain is the main hazard. For further details see https://www.metoffice.gov.uk/weather/warnings-and-advice/uk-warnings