Earlier this year, Terminal 5 at Heathrow wouldn't let me opt out of the scanner. Kinda felt like a personal violation, but the duty guards just said I was uneducated for having any concerns outside of their training.
Whenever I travel to a country that uses body scanners, I carry a printed copy of the airport's FAQ page (if available) and the government FAQ page or law that says that passengers are allowed to opt out. Then I know that I can show this to the people operating the checkpoints or to their supervisors if necessary. So far it was only necessary once (in the Netherlands, where body scanners are used routinely and some checkpoint staff aren't trained well about the opt-out option). But I still have a printout of the UK one and remember very clearly when the policy changed (prior to the change, I took the train through the Chunnel to Brussels instead of flying out of Heathrow).
Maybe I can set up a web page that has links to the policy and law documents so that people can find them more easily. There are already some pages that try to track which airports have body scanners, but I don't think I've seen one that tracks "what to show checkpoint staff to convince them that their country allows opt-outs".