[anghyflawn]

I suppose it varies. I am a non-EU citizen who lives in the EU, so most of the time I come to the UK on EU flights. Situations like the one you describe happen, but more often than not you actually have three planefuls of EU citizens crawling through the control while the handful of people who do need a visa breeze through. Although obviously you will have more non-EU people at Heathrow than at Liverpool.