When I passed my test admittedly sometime ago, I was told that the way to join a motorway or dual carriage way was to either increase or decrease your speed to the flow of the traffic already on the motorway/dual carriage way.
These days it seems that vehicles joining expect the vehicles already on the motorway/dual carriage way to either change their speed or pull over into another lane to allow them to join, (not always possible anyway) has the law/rule changed?