This wouldn't be an issue if all these 'cultures' assimilated into British/English culture and nationality

The Indians and a lot of the Caribbeans managed to do this with little issue (in fact the British welcomed them as they finally got food with flavour too lol) but the problems came when it was Africans and Middle Easterners arriving, carving out their own enclaves and holding onto past grudges and mentalities. They should have been rejected from staying.