Loading and unloading guidelines at the bottom of this page.

LGV drivers are restricted in their hours of work, and need to be able to load and unload to get the job done within their allowed hours. If the driver is doing multi drops than he must do them in the order that they are loaded as he can't get past one delivery to get to another.
So he does not have the option to go to his next drop and come back when he can get parked, in addition his next delivery could be many miles away.
While I sympathise with the plight of the motorist, there is another side, and if all jobs were as legally controlled as that of a commercial driver, there would be a lot more unhappy people out there.
Giok