P wrote:
(When I took my advanced driving test, I was told off by the police class 1
driver in the passenger seat for indicating when moving into a left-hand
lane, and the above was his explanation.)

OK, I'm not IAM (spit!), but RoSPA motorcycle trained, but the thing
about not indicating when moving back into the left-hand lane is
qualified by the proviso that you indicating won't help warn another
road user of your upcoming action - in this case it obviously would
have.