You do what makes sense. Which is not to crash into the car or truck joining from the slip road. It is correct driving (as well as simple courtesy) to look for vehicles wishing to join the main carriageway (especially large HGVs) and help them as much as you can, either by signalling right and moving to the right (if you can), or by lifting off or braking slightly (if you can't) and even giving the driver of the joining vehicle a brief single flash to tell him you have seen him. Obviously if you can't reasonably do any of that, don't do it.