Rule 259 of the Highway Code makes this the HGV’s fault; yes we want to keep our momentum up and yes common courtesy says the car should move over (there was no danger in him doing so). But, there’s no legal obligation for the car driver to move over a lane.
This is one of my top five pet hates about modern driving, people not knowing the rules of joining the main carriageway. Dense car drivers expecting trucks to move over being the worst example, without fail they have no clue as the what’s on the right hand side of the truck.