Have you got active cruise control active by any chance ??

That's my thought too as adaptive cruise control will adjust the speed.
I guess it's possible it's a safety feature based on wiper speed and set speed, not that I've had that with my previous corolla and current one.

Hi Mr. Lewis, thanks for your response. Cruise control was not activated and no warninig signs appeared on the screen, but dashboard/ speedometer lights went off. and the car was struggling.

Hi Mr. Jones, thanks for your response . Cruise control was not activated and no warning signs appeared on the screen, but dashboard/speedometer lights went off, and the car was struggling. ECO MODE sign appeared on the screen.

Hi Mr Jones, I forgot to mention that ECO-MODE SIGN appeared on the screen.

That sounds like it went into limp mode, certainly something for the dealer to look at!

Hi Mr. Lewis, forgot to mention that ECO-MODE sign appeared on the screen.

I cannot comment on the question/problem, but could you please take the time to tells us all what the dealer says about your problem after they have diagnose it.

Hi Mr. Thomas. Thanks for your response. I will certainly update you after the dealer inspection.

Hi Mr. Thomas, I took the car to Toyota main dealer [ MARSHALLS ] in Bristol and left it for a day
They tested everything including road test, but could not find anything wrong. They even could
.REPLICATE the fault. Will have to wait till the fault comes back in heavy rain.

Lets hope it was a one off thing,never to occur again.Thank you for replying.