A few points to consider...
'Being touched up':
If you were asked to search someone I can ganantee you would avoid any intimate area, it's human nature. Therefore, if you have something you shouldn't, where would you put it?
An example, a lady who worked for a very big airline at LHR was used in anti terrorist excercies when I was there, she'd had a breast removed. She used to hide a granade in the 'empty' side...and guess what? It was virtualy never found! And she was searched by trained police officers.
There is a very fine line between a sexual assault and a search that has to include parts of your body you don't want strangers touching. Yes, they may go over things more than once, possibly because they may have thought they felt something that shouldn't have been there.
Searches when the bleep doesn't go off...The bleep is irrelevent, the DfT dictate how many people in 10 have to be physicaly searched (no stipulation is made for crew/pax/other airport workers). Those searches are at random.
Most people wear a watch, a other bits of metal about them which can cause the detector to go off. Others have metal plates in limbs (some of which do set off the detectors)
No, the object of searches is not to detect DfT inspectors. The idea of the inspectors is to make sure the searches are being carried out to a standard. Similar to whats been said, but the idea being that the search is able to find most objects. It wont find anything, nothing short of a strip search and a rubber glove will do that.
Unless something has changed in the last few years there is no right or power as such to search, however it is a condition of going airside. There in lays the answer, if you don't like it....
Oh and for information...I went though MAN last week. I set off the detector. I was searched, asked to remove my shoes, made to walk though the detector again, then again. eventualy they were happy and off I went. Did it bother me? No, not really, I'd rather that, than find myself freefalling from 20,000 feet.