Having checked, it turns out that it wouldn't have mattered what I wanted - there is no right to opt-out in the UK.
The image produced wasn't one of the ones that produces a naked image - the display showed a cartoony image of a person, which could then have something superimposed over it to show anything it picked up. Which, in this case, it didn't.
I _think_, although I'm not sure, that the scanners are different from the millimeter wave ones that are now banned in Europe - the last paragraph of this article seems to indicate that in any case.
Original post on Dreamwidth - there are comments there.