So whilst I agree with you Jenny concerning the issue of boxes (entering is all to do with your exit being free), you should not use your horn for this as you are stationary (see below) ... Having said that, I would probably do the same:)

"A horn should only be used when warning someone of danger, not to indicate your annoyance at a manner of driving.

A horn should not be sounded when stationary on a road at anytime, other than at times of danger due to another vehicle on or near the road.

A horn should not be used on a moving vehicle on a restricted road (basically a road that has street lights and a 30mph limit) between the times of 2330hrs and 0700hrs."