the head of a queue is at the end point, so all the organisers have to do is decide in advance where they want the queue to go and direct people to the current tail end, just as they do in Disneyland. It shouldn't matter whether you arrive from Polperro or Norwich, but it's important that the current tail end be publicised so they know where to go.
They've been planning this London Bridge Is Down thing for years. I'd be surprised if the planning included "Oh, just let people queue anywhere they feel like."