If there is a line at LHR immigration, I often make a mental game of guessing who ahead of me will not be out of immigration before me. I've gotten rather good at it -- if I can see the passport cover, parts of the landing card or if I can hear the accent/language/dialect being used.

I often see young women (whether single or in pairs) from South America, the former Soviet Bloc or Asia being singled out at LHR immigration on Thursdays, Fridays and Saturday mornings. Single American females not there on business usually get asked far more questions than single American males regardless of the purpose of the visit.

Recently married women arriving into LHR by themselves -- especially from Asia -- seem to get quite the questioning too. But generally not so much that they get sent to one of those "rooms". LHR also routinely seems to send some people for a "medical check"-type thing at immigration too. I haven't witnessed that zoo at US airports thankfully. (The closest thing I have seen as a passenger is ORD having some health clinic facility available to people who are departing for either domestic or international flights.)