If you're selected, scanning is mandatory.
From the code of practice:
Communications will be available at the security screening area to inform passengers that .For the benefit of all passengers' security, passengers may be required to be screened using body scanning equipment.
It should be made clear at the earliest possible stage that all passengers selected for screening by a body scanner must be scanned. If a passenger declines to be scanned that passenger must be refused access to the Restricted Zone, with the result that the passenger will not be able to fly.
So you now /have/ to submit to being scanned or not go through. You can't even opt for a pat-down.