The EU policy is crystal clear:
passengers must be given the possibility of opting out from a security scanner. In this case the passenger shall be screened by an alternative screening method including at least a hand search;
The UK government, as usual in its bizarre anti freedom ways, tried to resist opt outs going so far in 2010, per this Guardian article
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.
but as of November 22, 2013 you can opt out at every UK airport as well. However, if you do, they will make sure to make your life miserable: in 2017 August, I was picked for the full body scanner at Heathrow T2 and after opting out they have removed and inspected every single piece from my carry on. This was a lot of fun as I was travelling for six months and there were a lot of pieces. I have no idea what happens if you are late from your flight because of this charade, I caught my flight, even if barely. It took about 30-35 minutes.