Finally had a response from the famously flaky and halfhearted West Berkshire council.
“the majority of LED street lights installed in West Berkshire have a colour temperature of 5000K. Dimming takes place between Midnight and 05:30 and the level of dimming depends on the location and road type, but in general the reduction is around 25%.”
I have to say that I’ve seen no evidence of dimming but at only 25% I doubt I’d notice.