On the 22nd June 1948 the Empire Windrush had docked at Tilbury in Essex and the official history of Britain prior to this date is thus defined as B.C. (Before Colour).
This is an inaccurate picture of British history and quite why it has become part of historical fact is an interesting question On the one hand it is about romanticism, like the America's Mayflower bringing pilgrims to a new world and the birth of a new nation. On the other hand it is cynicism, an all white world that was polluted on that day in 1948 by an impure influence.
It is also myth that the 1958 Nottingham/Notting Hill Riots were Britain's first ever race riot, as there was a reported race riot in Liverpool two weeks prior to the docking of the Windrush. In 1919 there were also race riots as 'the local white population clashed with black workers and seamen, many of whom were left unemployed at the end of the war. In Cardiff, in particular, white ex-servicemen, including Australians stationed in the area, headed lynch mobs that terrorised the city's black community during a week of violence that left three men dead and dozens more injured'. (http://nationalarchives.gov.uk/pathways/firstworldwar/spotlights/demobilisation.htm)
The history of non-white migration within the U.K., although commonly believed to be a post second world war phenomena has a longer history. Many port towns and cities in the U.K. have had a pre-war non-white population, for example Liverpool, Tiger Bay in Cardiff and the East End of London. The latter (and not Soho in the West) was the site of London's original 'Chinatown' and has had a Chinese population for over 200 years. There were many reports of hostility towards Chinese migrants based in the East End in the 1800's who faced assaults, racist attacks and jibes that the Chinese were dirty and immoral.
A common accusation levelled at the East End Chinese were involvement in white-slave rackets and vice dens of Opium and gambling though 'the real Chinatown bore little resemblance to the lurid underworld of the hack writers. Opium smoking and gambling certainly did take place, but most people in Chinatown were too busy trying to make ends meet' (http://www.portcities.org.uk/london/server/show/ConNarrative.96/chapterId/2682/The-port-in-literature.html).
Along with the Chinese inhabitants, Indian servants and nannies known as Ayahs had settled dockside because 'if they could not secure another job with India-bound passengers, they had little option but to remain in London' (O'Neil 1999:49). There had also been 'dark skinned' sailors of various backgrounds such as Somalia, Yemen, and the South Asia collectively termed 'lascars' who had 'jumped ship and stayed, putting down tentative roots which, in some cases resulted in the organic growth of a close-knit community such as that in Limehouse' (O'Neil11999:49). Lascars were taken on by the East India Company because like the Chinese and Irish immigrants of the East End, they would do hard, dirty work for lower rates than British seamen.
In earlier history of migration there had also been examples of inter-group fighting between non-white migrants, it was reported in the East End of London on October 12 1806 that there had been a riot between 300 Chinese and a group of Lascars and Irishmen that numbered around 150 (O'Neill 1999:54).
Black residents prior to the windrush had also been in positions of power, including two queens of England. Queen Phillipa of Hainault who reigned as Queen Consort from 1328-69 (http://100greatblackbritons.com/bios/queen_phillipa.html). Also Queen Charlotte who reigned as Queen Consort from her marriage to King George III in 1761 to 1818, who was 'directly descended from Margarita de Castro y Sousa, a black branch of the Portuguese Royal House' (http://www.100greatblackbritons.com/bios/queen_charlotte.html). Queen Charlotte is also the Grandmother of Queen Victoria and thus blood related to the present day monarch.
And although never a resident of England, its patron saint - George of Lyddia - was also non-white. He was born in Modern Day Turkey and was of Palestinian origin.
So the influence of non-whites in Britain was felt long before 22 June 1948.