Whilst I sympathise, and this absolutely boils my ****, and I think these people should be fired into the sun, it's kind of on you and the three cars behind you as well to make space in this situation.
There's a similar single-lane slip road to join the dual carriageway near me, used frequently by heavy lorries from a nearby quarry. By the end of the slip road (1/4 mile or so), they are often not able to reach more than 40-50MPH before joining the DC. Yet often, I see other cars join the slip-road behind one, then sit right on it's rear bumper all the way, before joining at the same speed, and then out of frustration, cutting across straight into the outside lane.
When I get caught behind one of these lorries at the top of the slip road, I just slow right down and hang back until they've joined the DC. That, way I leave myself as much room for acceleration on the slip road as possible. Same goes for OAPs; you can often tell when someone's scared of their accelerator pedal. Better to hang back and not put yourself in the same danger.