Something occurred to me while reading about mestizos and mullatoes in Brazil. It seems that during the period where slavery was widespread, the idea that blacks were inferior and hence well suited to agricultural labour abounded. However, after slavery was abolished, the rulers of Brazil realised that they would have to mobilise a large mass of people for work they were generally considered to be unsuitable for. So they propagated the idea that a mixed race industrial labourer was superior to both black and white industrial labourers. So maybe you could have a Britain which suffers terrible internal ructions, possibly a worse English Civil War. Many Britons flee to the colonies, and don't come back. When the Restoration comes around, there is a severe labour shortage, so they bring in Africans and Indians to work the fields. Slavery becomes commonplace, up until the 1850s, as opposition to slaver domination of the economy builds up. After that, the same thing happens as in Brazil, beginning a period of interracial relationships. This makes Britain a far more mixed place than it is now.