Jubilee.
You are right about Schengen (except that it does apply in all three Scandinavian countries, Iceland, Switzerland and the rest of the EU except GB/IE)
However I don't think this explains the non-mixing of passengers. Take T5 at Heathrow as an example. A passenger in transit through T5 is separated from departing passengers until such time as s/he has cleared security. S/he is then free to mingle with other departing passengers and has not cleared UK immigration.
However I think a passenger arriving at T5 off a domestic flight doesn't get security screened again.
It would seem that the UK doesn't trust other countries security (even though some are more rigid). It doesn't want to take the risk of some black-hats beating the system - although IMHO it would be very easy to separate only flights from countries with poor security, if there are any.