Good old Wikipedia gives us a starting point - lots here I've not yet learnt about: https://en.wikipedia.org/wiki/Racism_in_the_United_Kingdom
What racism have you actually seen or experienced? What were you specifically referring to for example? That link was very general eg. you could say that you think US police are institutionally racist.
If we're known to be so racist in The UK, why do you suppose millions for foreigners from Europe, Africa and Asia are desperate to come and live here? Thousands risking their lives every year to enter.