Because it is against the law not to have a government ID on your person. You can get arrested for vagrancy among other things, depending on the cop and the situation.
He's in the UK not the US he does have to carry ID. Technically he should have a licence with him when driving but in practise if he got pulled over by the police he would be given a 'producer' and he would have to present his documents at a police station of his choice with a certain number of days.