At least you have a conscience about doing it, it’s more than could be said for some drivers.
I’m not a fan of flashing headlights, not a smart idea for a fully liveried truck to be flashing it’s lights at a car driver who is driving perfectly legally either.
I was on my way to work in my car and a truck from a reasonably well known local company was tailgating and flashing me on the A1M; there’s a long, straight two-lane section with chevrons and signs saying “keep two chevrons apart” and that’s just what I was doing. The daft thing was the right hand lane was quite a bit less busy so after a minute he pulled into that lane and started tailgating someone else 
About 10 miles further on I pulled off the motorway and he was no further forward than he had been earlier and still tailgating someone else.
As it turned out one of the guys at work is a good friend of the TM at the company this driver works for, he said he’d pass a message on to the TM to have a quiet word with the driver, but if I’d been a regular member of the public there’s a good chance he’d have DVSA on his case for driver conduct issues.