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...
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.