Not much experience with LHR, but another North London airport with security staff's obsession to make life miserable for people.
Once I kindly requested not to remove my watch from my wrist, they was not happy, but excepted it, as I told them I knew the watch would not set it of!
Of course, I was right, passed the security perfectly. However the assistant manager come over to me, and told me I should not refuse security staff request. I told him that under no circumstances did I want to remove my watch, as it was of high value!
I was happy to be scanned if it set of the alert.
Anyway then the ingenious security staff told me, I had to remove it because it was an electronic device! It had to be, for the watch to work.
I kindly declined, and told him the watch was not an electronic device, in fact it had no electronic components inside, as it was a mechanical device.
By now this assistant manager started to get really agitated and blowing himself up, so I nicely explained him, that this was a mechanical watch, and if he was not aware of that this was possible, maybe he should educate himself a little before make such statements.
By now he wanted on this basis to refuse me entry, took my passport and boarding pass away from me, at this moment his boss come over to him, and wanted to know what was going on.
During this whole process I had not even bothered to get agitated! However some sense was seen by his boss, and I was allowed to enter trough the security!
This is the same security guard who gave a lecture to crew because they did not have sealed bags for liquids, and forced us to pay £1 for a a bag to get trough security with our tooth-paste!