More than one-and-a-half inches of rain has fallen in 24 hours, with more wet weather forecast for the coming days.
The Met Office has issued a yellow severe weather warning for rain while the Environment Agency has four flood alerts in place, meaning flooding is possible - three in the South East and one in the Midlands.
Northern parts of the UK could also see heavy downpours at the end of the week as Hurricane Cristobal, which has hit the Bahamas, crosses between Iceland and Scotland although conditions are not expected to be as "intense" as when the remnants of Hurricane Bertha reached the country earlier this month.
Herstmonceux in East Sussex recorded the most rainfall - 1.6 inches (40.8mm) - between 9am yesterday and the same time today, according to Met Office figures.
Many other places also experienced similar levels of rain, with the Bank Holiday weekend a washout for the south east of England in particular.
Met Office forecaster Dan Williams said: "It's been a pretty wet 24 hours. It sums up a fairly wet Bank Holiday Monday across most of the UK, which is extending into today with quite a few showers around in the South. But there is some really nice weather to be had in northern parts of England, Scotland and Northern Ireland.
"Tomorrow looks to be a dry and bright day, particularly in Scotland, Northern Ireland and eastern parts of England, but it will be cloudy and wet in the South West and southern parts of Wales. Thursday is not too bad once the rain clears up, with showers and sunny spells."
Meteogroup said 38mm of rain (just under 1.5ins) was recorded within a 24-hour period at St James's Park in central London, which is unusual for winter, let alone August.
Forecaster Billy Payne said: "We do get these very wet days in summer sometimes, but it's not too often we see widespread heavy rain like that over a whole day."
Although the wet conditions have brought temperatures up slightly, there is no sign of any last blast of summer heat as August draws to a close.
Mr Payne added: "It will be warming up a little bit, to the low 20s. By the middle part of next week the weather will have settled down and we may experience some high pressure, but there is not likely to be a heatwave."
He added that average temperatures for August were 1.3C below average, unlike July which saw long periods of hot weather.
Mr Payne said the effects of Hurricane Cristobal might be noticed in Scotland late on Sunday as it passes above the UK, but will "not result in dramatic rainfall".