Racism - Differences in "race" in the US and UK

It always perplexed me, that most census forms here in the UK, or Equal Opportunity forms given as part of a job application, list not just "White", "Black", "Asian" etc, but go right deep into the ethnic background of the applicant. Asian include Indian, Pakistani, but sometimes Chinese is its own group. Black include Afro-Caribbean. White includes English, Irish, Scottish and Welsh. I always choose "White: other" and handwrite "American". This is most likely because the definition of "race" in the UK includes "ethnic origin".

The law defines racial grounds as including race, colour, nationality or ethnic or national origins. (Equality and Human Rights Commission)

In fact, this sample form doesn't even use the word "race", it uses "ethnic background". Why is it that only Blacks are "British" here? This other form also uses the term "ethnic origin", but noteably combines all white people into either "British" or not-British. All of the other categories are broken down by country. Yet the form says that "Ethnic origin questions are not about nationality, place of birth or citizenship. They are about colour and
broad ethnic group. UK citizens can belong to any of the groups listed." What does that mean? Aren't all UK citizens just "UK"? (or whatever British term they choose.) I've even seen the word racism to apply to discrimination against "gingers" (people with red/ginger hair).

Part of my confusion stems from the American look at racism. Ask most Americans what the different races are in the US, and you'll hear White, Black, Hispanic, Asian and (in some places) Native American. This can be seen in the list of references on the Wikipedia article on racism: "A poll on black, hispanic and asian americans on race relations", "red, white and black", "where black and brown collide", "both white and red". Ask people for an example of racism and they'll mention black people. Ask what defines race and they'll say the colour of your skin.

It isn't strictly true that race in the US is only colour. The US Constitution's 15th Ammendment (1870) reads:

Amendment XV
(Ratified February 3, 1870)

Section 1

The right of citizens of the United States to vote shall not be denied or abridged by the United States or by any State on account of race, color, or previous condition of servitude.

Section 2

The Congress shall have power to enforce this article by appropriate legislation.

Race and color are separate items! Yet you'll often hear the phrase "racial and ethnic minorities" such as over here are the Racism Review blog. (In the same blog, the author uses the phrase "colorblind" as an opposite to racism.) In their recording of Hate Crimes (here, 2005), the FBI distinguishes between race crimes (black, white, native, asian) and ethnicity crimes (hispanic). Technically speaking, hispanic is not a race in the US. Yet crimes against hispanics can still be called racist.

Here's my conundrum: Race is biological. I am white because my parents were white, and there is nothing I can do to change that. I am American, though, because I was raised in America. While it was not a choice offered to me at birth, I do now have to choice whether or not to join another white ethnic group, such a converting to a different religion, or changing my citizenship. If I changed my religion or citizenship, I would still check same US census box. Would my British census box change? Would I call myself English, as I'm marrying an Englishman? Or would I be Scottish because I live in Scotland? Or would I still be American as I speak with an American accent?

In a sense, I suppose I'm not the only one. This blog asks people from Taiwan to write in TAIWANESE on the census forms instead of checking the Chinese box. The difference between "race", "colour" and "ethnic group" varies in other countries as well.