YOu can say exactly the same of all other road users. The type of vehicle is irrelevant, they don't drive themselves - it's 'normal practise' for that particular person in control of the vehicle. Before I was a biker I often used the generalisation 'bikers are all idiots' now I'm older and wiser I only say 'the UK's roads are full of idiots driving different things'...fair? Yes.
As for the original post, sounds like its not the wisest scenario that biker is putting himself in. I remember my 125 days when on the motorway/A-roads with a lack of horses - HOLD BACK and be defensive - inside lane was absolutely fine for me and overtaking the 60 mphers was done with foresight.
Not sure the Police would do anything if you did make a report as no crime or incident has occured.
I'd even be unsure whether this biker would get pulled over by a traffic cop if he witnessed what you did, these days.
All I could suggest is to stay clear so you don't get caught up if something did happen.
Personally I'm thinking of getting a dashcam for the car as the UK's roads are getting wilder and wilder and if something did happen a video would be much better than a written description.
