I have noticed that a lot of british people have dirty blonde hair,why is this?

Generalization of this magnitude is just ignorant. When you use the word British that is a lot of people and considering the country was invaded by the Jutes,Saxon, Angles, Vikings, the Danes the Romans, the French and was originally home to the Picts before the Roman invasion. So perhaps you may have seen some British people with 'dirty blonde hair' have you thought that after the color black blonde is one of the most widely used hair colour? Very few individuals have actually found to have DNA that linked them to any Angles or Saxon links.