I've noticed a number of car's headlights seem to vary in intensity as they go along, maybe slowing/accelerating, I'd have put this down to a faulty regulator back in the day but these are fairly new cars and I doubt it's a fault, any idea why they do it?

That's a phrase I often think of when they are dazzling the hell out of me in the rear view mirror

I'm sure it begins with F not sure it's fluctuating. I had one come up behind me at a T junction which was so bright I couldn't see out of the front or the sides, the light went through my car and blinded me off the hedge opposite.

In that situation I'm not too concerned about the spelling

Might it be automated self-levelling on the lamp unit taking a while to catch up with the car's attitude?

Or matrix headlights, which do not respond instantaneously.

I'd put it down to the attitude of the drivers, though some times the altitude of commerical vehicles can impact, with the number of 4x4s round here it's often both!

My DS7 with LED-vision adapts the intensity of the lights according to speed. Driving slowly makes the beam go wider so you get a good view of the road and the what's on the side. Drive faster and it focuses on the road further ahead. Works a treat.