I took my HGV test last year and you most certainly do get told not to do it.
You do not indicate to overtake parked cars, the following and oncoming traffic expects you to drive around it and make progress.
You get told not to do it on your motorbike test, on your car test and on your HGV test.
I always do it if i have cars infront or behind me. Cant remember if your allowed on your test, i just think if someone is behind me or infront of me they need to know im moving around the car infront.
Road users behind me would indicate to,, it lets people know there is an obstruction infront and you have to move onto the other side of the road to get past.
Then, im approaching a parked car, i will indicate to show the oncomming car i am not waiting behind it for them to pass first.