Earlier quoted context omitted.
I can say that I was required to use one to get back into the U.S. while transiting through Schiphol back in October 2013. No opt out, just the scanner and "I'm not forcing you to fly today - you can stay here." Pretty rude, too. Since then, I always transit through Heathrow. It's a logistical nightmare with all those buses, but they've never coerced me to go through a scanner. Those biometric checkpoints, on the oth…
I don't know why you say that, while there are not scanners on every security lane at Heathrow they _cannot_ be refused, there is no pat-down alternative like in the US. https://www.theguardian.com/uk/2010/jan/26/new-body-scanners...
From Heathrow's security FAQ page (http://www.heathrow.com/more/help-with-this-website/faqs/sec...):
The only alternative that can be offered to a scanner is a private search which allows for a more extensive hand-search than usual. Passengers will be escorted to a different location in the airport from the main search area (eg a private search room). The private search may involve the loosening and/or removal of clothing. A person undergoing a private search may ask to be accompanied by a witness.
This alternative screening method will take significantly more time than passing through a security scanner.