You say this as if the UK hasn't always been multicultural. Welsh, Scottish, Irish, English, there was cultural issues for time immemoriam... Normans, Anglo-Saxons, Pictish, Caledonian, Cornish, Bretons, Celts, Romans, and so forth. Cultural clashes have existed long before different colours made it easy to identify. I garuantee you that if all the people non-indigenous to the Isles vanished one day, it wouldn't be long before all the nasty hatred reserved for 'Pakis' was redirected at something else, Catholics, Protestants, northerners, Irish, Scots, etc. Racism is just a funnel that people use to funnel the blame, hatred, and evil into; it's very easily redirected into something else.
People like to believe that time and progress is linear, you say 'this day and age' as if we are meant to be more enlightened as a species than 500 years ago. Well we aren't, we just manifest our evil differently.