Body scanners have been introduced at Heathrow in line with UK Government requirements which state the following: "For the benefit of all passengers' security, passengers may be required to be screened using body scanning equipment. Screening will be conducted by security officers acting on behalf of the airport operator. Images of passengers will not be saved."
This has been introduced to improve security and refusal to comply will mean passengers cannot fly.

Body scanners will give airport security staff an increased ability to detect explosives or other potentially harmful items hidden on a passenger’s body.
For the present time only a proportion of airline passengers will be selected for screening. Passengers will not be selected for screening based on their race, ethnic origin, gender or age.
Safeguards have been put in place to make sure that privacy is respected.
However, if a passenger is selected for scanning, and refuses, they will not be allowed to fly.

Earlier this month two women, one a Muslim, became the first people to be barred from boarding a flight at Manchester airport because they refused to go through a full-body scanner. The women, who were booked to fly to Islamabad with Pakistan International Airlines, were told they could not get on the plane after they refused to be scanned for medical and religious reasons.