I believe this was previously the case, but it is now possible to opt out. I flew from Heathrow (Terminal 2) on Friday, refused the mm-wave scanner and had a pat-down in a private room instead.

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.