That has enriched everyone's culture and lives. But workers are also forced to compete with one another on the labour market for resources. This provides a seedbed for turning differences between groups of workers into intense racial divisions. Employers can fuel this scapegoating by using immigrant workers to undercut indigenous labour. Victorian capitalists deliberately used Irish workers in Britain in that way.
Capitalists can also scapegoat whole groups of immigrants.
This does not mean that all the employers sit down together and plan a racist conspiracy. But firms will often employ workers on a segregated basis. The harassment suffered by Ford worker Sukhjit Parma exposed the way management encouraged racism at its Dagenham plant. And there are always establishment figures who seek to whip up racism. So the history of Britain is a history of immigration, racist scapegoating and resistance to it.