(Oddly, I got a 404 on your link but when I start at the MAN airport main site and through "security" to the page on scanners, I end up at the same URL you provided. Weird.)

To the OP: as janetdoe's link says MAN is now MMW, here's what I would do if I was pregnant and had to fly out of the UK.

1. I would try to avoid the scanners on general principles (privacy, dignity, speed, being unable to see my belongings). This would mean taking a moment to survey the setup before choosing a lane as far from the scanner as possible, and making sure that I don't have anything that would set off the WTMD.

2. If randomly chosen for the scanner anyway, I would mention that I'm pregnant and hope that I can get excused. (The website suggests that they're not too sympathetic to this approach, and if your wife doesn't have a visible baby bump it might be difficult.)

3. If I had to go through the scanner after all, I would NOT worry afterwards about any health danger from MMW. The worrying itself would be far more dangerous to one's health.

Congratulations and I hope you have a pleasant and non-eventful trip!