It is, however, a matter of common courtesy to allow a car to enter by either slowing a fraction or moving to another lane if it is clear.
Of course in these days of poor education in all walks of life and high stress courtesy is not common any more.
We arent a lot better at sticking to the rules than the average Italian - we're nothing like the Germans or Ducth. That said there are clear Highway Code rules and they do put the onus on the car joining to wait until there is a gap rather than force cars already on the road to alter speed or position. Rule 259. And just like the IRPCS at sea, one of the purposes of the Highway Code is to give predicability. As with the priority at roundabout rules. So yes, whilst I agree with your views about courtesy and the lack of it in the UK, when it comes to driving I think the predictability issue is more important.
