There is no way that you would join the left-lane queue in this scenario unless you intended to leave the motorway. Since the queue is only for the left lane on the sliproad, in this case, 99.9% of the time it's probably perfectly safe to leave from the middle lane.
The only exception I guess would be if someone who had been queuing changed their minds and moved off to continue in lane 1. Even then, I would imagine if you were driving past the queue in lane 2 you'd be going sufficiently quick enough to easily pull in front of them.