Many people got out their cameras to photograph and video the electrical storm, which was called “utterly insane” and “like being under a strobe light”.
Others remarked that they had “never seen a storm quite like this” and said the flashes were “stunning”.
Another woman remarked it was “like a rave outside”.
BBC Weather presenter Tomasz Schafernaker called it the “mother of all thunderstorms” as he watched it over London.
“Oh boy! This utterly insane,” he said.
“I’ve never seen a storm with such frequent lightning in my life I don’t think. Mostly sheet lightning and not too loud but flashes are spectacular.”
Forecaster Gemma Plumb, from BBC Weather, said as the storms push northwards across England throughout Sunday, more are coming up over the English Channel from the continent.
The Met Office weather warning for rain is in force until 6:00 BST on Monday and covers all of Wales as well as southern and central England.
Flooding of homes and businesses could happen quickly, the Met Office said. It added that fast-flowing or deep floodwater is possible with damage to some buildings from flooding, lightning strikes, hail or strong winds.