That maybe so but common usage isn't aligned with what the Highway Code defines as correct usage.

Take lights for example:

+ flashing headlights on a dual carriageway or motorway is a way of telling a driver to pull across so they can pass

+ flashing headlights near an obstruction is a way of telling another driver that you're giving them priority

+ following on from the previous point, flashing back is a way of saying thank you to the other driver for letting you take priority (this is done after you've already started moving to make it clear that you're not trying to hand priority back)

+ flashing hazards briefly is also used to say thank you, albeit typically if a vehicle is behind you rather than in front

+ flashing hazards on a duel carriageway or motorway is a way of warning other drivers that you're breaking heavily (as break lights don't inform the spectrum of difference between soft breaking and emergency stops). At least in this scenario you are actually informing others of an oncoming hazard

I'm sure there'll be other colloquialism I've forgotten too

For flashing headlights, the highway code is so far out of alignment with the real world that it would be more pragmatic to change the highway code to accommodate it and properly define liability. Being predictable is the most important thing for safety and you can't be predictable while treating all flashing headlights as a warning.

Yes, it’s a bit ironic that the only reasonable use of a horn is often the least likely justification for honking by Western (or at least American) drivers.

As an aside, Japanese drivers flash their lights (both headlights and taillights) as acknowledgement for letting them merge or pass—like the dual behavior of honking a horn as an expletive.

That's common in the UK too. Rear warning lights that is, can't imagine when headlights would be in the right place for it; they're more often used by the other person in that scenario - 'after you'.

Here in the somewhat rural U.S., we tend to use the horn only when a driver isn’t paying attention when the light turns green and we’re behind them.

I’m in a city and over 90% of my horn usage amounts to “put down your damn phone; your light is green!”

Beware that flashing your highs in Japan can mean “GTFO out of my lane”, and some drivers get really mad about it