christep - don't doubt the theory, but on Saturday evening there was nothing voluntary about being herded into a roped off area, sheep like, and instructed to take shoes off for scanning. This just after passport control beyond initial security checks.
If voluntary rule applies throughout LHR, then I assume same applies in T1 and T4 where everyone has to remove shoes at first security check point.
If you are unlucky enough to depart from gate 27-29 in T1 you also have to go through the same procedure again - for whatever reason!
Work in the aerospace business and fully accept the need for security, but I just can't accept the way things are currently being done at LHR. It's neither consistent, from one terminal to the next, friendly, nor reassuring!