Who has right of way when cars enter a motorway/dual carriageway? I usually move to the centre lane to accommodate cars entering from my left but where there is no opportunity to do this am I obliged to give way to cars joining the inside across broken lines?

I am happy to give way to avoid an accident but many joining the inside carriageway seem to simply show a right indicator light and drive onto the carriageway without any consideration for those doing a higher speed who are already on the carriageway.