One of the biggest challenges for sensible drivers nowadays is that we share the roads with drivers who are arrogant or stupid or both, bend the rules, or sometimes are just downright dangerous.
In that kind of environment one often needs the patience of a saint to maintain courteous driving and to continue to drive defensively, without rising to the 'bait'.
Lowering one's expectations of other peoples driving is a good way to start.
Don't expect to get thanked for letting someone in. Don't expect to get thanked for virtually 'parking' your car so that someone can pass you in a narrow road. We have all done it and watched in astonishment when the other driver drives off oblivious to our politeness.
Anger is sometimes described as "Unmet expectations", so lower your expectations of everyone on the road and you are less likely to be angered by the idiots out there.
As a former DSA examiner we used to teach the "Two Second Rule" or four second rule if it’s raining (someone has already raised this in another post). The mantra we taught was to say out loud: "Only a fool breaks the two second rule", as its takes 2 seconds to say it. (Say it twice in the rain).
The problem in practice, as already posted, is that someone moves from lane 2 to lane 3 to fill the gap in front of you. Or more taxing (of your patience), they overtake on the inside and move into the gap. Illegal but common practice.
I have sat there on a busy motorway trying to maintain a 2 second gap only to find myself 6 cars back from the van I was previously behind, because people have barged in front of me.
IMO, expectations and anger management are important issues for the modern driver on Britain's busy roads.
Before I sound like a model driver, I'm not. I HAVE managed to lower my expectations and nowadays drive in a calmer frame of mind (as my wife will attest), but when I'm overwhelmed and about to 'lose it' (we are all human after all...) I recite the mantra: "There are more donkey's ars*holes in the world than there are donkeys" (!). It helps me ‘vent’ and thus relax, try it!
So I don't expect to marry theory with practice all the time. People will fill your 2 second gap. They will tailgate, and they will continue to treat their indicators as a factory fitted optional extra, like a sunroof. Expecting it helps, as does my DVR car cam that quietly records everything in front of me! (And soon a second to record what’s behind me).
Cheers,
Robert