This poem is a celebration of the history and cultural diversity of the British people. The speaker, as if describing the steps in a recipe, describes the different cultural and ethnic groups which have historically settled in Britain, and from which all modern British people have descended.
In the opening stanza, the speaker says, "Take some Picts, Celts and Silures / And let them settle." This is a reference to the Celts who arrived in Britain in approximately 500 BCE. Throughout the next four stanzas, the speaker describes the other cultural and ethnic groups that have settled in Britain since the Celts. He mentions "Romans ... Saxons ... Jamaicans ... Pakistanis ... (and) Palestinians," and describes them as if they are ingredients added to "the melting pot."
At the end of the poem, the speaker notes that "All the ingredients are equally important," and that "Treating one ingredient better than another will leave a bitter unpleasant taste." Here, the speaker uses the recipe metaphor to make a comment about racism. In Britain there has been, in recent times, a rise in support for various white supremacist organizations, such as the National Front, the British Nationalist Party, and the English Defence League.
Zephaniah is, in this poem, pointing out the absurdity of white supremacism by highlighting that the British are all descended from immigrants. He is also celebrating the cultural diversity in Britain, and calling for all races, ethnicities and cultures to be treated equally, so that Britain, like the metaphorical recipe, can be the best it can be.