A flooded office in Bridgend
Heavy rainfall is expected to move northwards after the south-west, south, and south-east parts of England, and south Wales battled with flash floods.
The Environment Agency has two flood warnings and 38 flood watches in place in England. There are five flood watches in Scotland.
In Bridgend, floodwater in one street reached a depth of 3ft (0.9m).
Southern Scotland and Northern Ireland are expected to bear the brunt of Thursday's bad weather.
BBC weather forecaster Rob McElwee said up to 50mm (2in) of rain could fall.
He said: "The wet weather will be in southern Scotland, with the potential for flash flooding there and possibly in Northern Ireland.
"It will be a sharper line of heavy rain than we saw yesterday, and much dryer in England and Wales."
Areas also expected to be hit by downpours include northern England and the Midlands.
Places already hit by heavy rain include Cornwall and Oxfordshire - both of which have seen rainfall reach 10mm (0.4in).
South Wales saw 23mm (0.9in) of rain between 0600 and 1200 BST on Wednesday.
The day saw some parts of the South West receiving 50mm (2in) of rain - four-fifths of a the amount in a typical month.
The AA attended 28% more breakdowns than normal - mostly weather-related home starts - with the busiest regions being the home counties, London and the West Midlands.