I think people maybe have a rose tinted view of what Britain was like in the past and that if we had more of a monocultural like in the past then the good old days would come back.   I don't think they do.

Spoiler though - there were no “good old days”.

Infant mortality was 10 times what it is now.  Advances in science have changed this, nothing to do with monoculture being bad.
People died of TB in their thousands.  Advances in medicine/science have changed this, nothing to do with monoculture being bad.
Real poverty was widespread.  We became more civilised and compassionate, plenty reasons for this but no evidence of it being our enrichment due to multiculturalism.
Children were battered senseless at home and in schools.  Not always, but again there's nothing to show this was because of a monoculture.
World Wars occurred.  Yep, not a fault of a monoculture.  In fact, nowadays with society being so fragmented, would our people really have come together to fight WW2 for example?
Nuclear weapons were dropped. Not the fault of monoculture.
People had to have surgery without anaesthetic.  Aye, because we didn't know how to do it?  This wasn't a fault of monoculturalism though?
Huge swathes of the population had little to no education.  Until the reformation!
Crime rates were through the roof.  A blanket statement for a subject which has many complexities.  The idea that multiculturalism has helped stop crime is laughable though

I could go on and on and on.

The good old days were really pretty shite.

The above points aren't down to us having a monoculture though.  Please see my text above in red.

Spoiler - No one is saying the old days were better than now, but that doesn't contradict my points that immigration/multicuralism/diversity are more negative than positive.