Nevada I think what you don't appreciate is that Britain used to have a great many colonies - even as recently as the fifties and sixties. The people who lived in these colonies were considered British "Subjects", which is close to being a citizen. People of all races in the colonies were once allowed to apply for and be given British passports. It was once quite easy to emmigrate to the UK from Nigeria, Canada, Zimbabwe, Uganda, Australia, India, Pakistan, Bagladesh, well you get the idea.
Things have been tightened up somewhat recently but British citizens of say Pakistani origin tend to marry people from Pakistan. Also they can bring in close family members such as parents to live in the UK. Plus the dismanting of border controls in the E.U. mean it is very much easier for illegal immigrants to enter; and they don't fill in Census forms!
The UK - England in particular, is a very diverse population, simillar to the USA but with more people from south Asia. Please visit Bradford next time you are here.
Also as I said once before in a thread about Kosovo, the demographics of a country can change very quickly if one group is simply outbreeding another. It happened in Kosovo with the Albanians rapidly displacing the Serbs, it's happened in the Lebanon with the Christain Lebanese being outnumbered by the Muslims (although the displacement of the Palestinian people caused by the creation of the state of Israel has probably had more effect there) its even happened in Northern Ireland where the Catholic population has grown from 34% to over 40% in the last 30 years.
So my point is that we in the UK have experienced, and continue to experience, considerable immigration from Africa, Asia, and the Middle East.