when in full uniform
. Rightly or wrongly it doesn't matter.
The screener is correct in saying that he must open the bag although you can refuse and not be allowed through the screening point. The procedures for this are not contained within the act or regs but in the Aviation Screening Notice which is not made publicly available.
As for your rights, you are entitled to be treated with courtesy and respect but under the current regs, not differently to everyone else entering the sterile area.
This sign introduced a few years back covers it quite succinctly
"A person at this screening point is taken, by law, to have consented to undergo a screening procedure (except a frisk search), unless the person refuses to undergo the screening procedure. A person who refuses to undergo a screening procedure will not be allowed through the screening point."