As all have said, UK customes come out for full inspections of flights arriving from places of high drug traffics into the UK (or EU). I was asked once about my arrival flight and let go around when they were checking a flight arriving from Jamaica. I was arriving from USA. They were hand searching every bag. Prior to that occation, I always wondered why they had so many tables with no staff on sight.
It is similar in France. They only do searches on selected flights. It is not a matter of a tip off... just based on arrival from city.