In the ice cream van scene, Gene Hunt makes a quick hand gesture, which results in the children crying. In the UK, that gesture (known as "flicking the V" or "Two's Up") is considered offensive and has the same status as the middle finger.
The offensive hand gesture that Hunt makes to the children is known as "the fingers". It is thought to originate from English soldiers soldiers fighting the French at the battle of Agincourt in 1415, where they would taunt the enemy by holding up the two fingers they use to shoot arrows from their longbows - a superior weapon to the French Army's - to show that they hadn't been captured and had them chopped off.