Ok I’ll try and give a reply
the reason we have high immigration is because UK born and bred people tend to not want to do minimum wage jobs. There are plenty of minimum wage jobs out there but many people will get more in benefits than if they take them so completely understandably don’t! Also it is very hard to live on minimum wage jobs if you are in one
we want to improve the education system to everyone, no matter the background, gets a good education which is a very admirable aim but once you have worked hard for your A’s you are not going to be interested in being a cleaner for £10 an hour are you?
so ultimately we will always rely on cheap external labour for the low paid jobs . The government will always have to import those roles in. And so the cycle continues .
also having more children per family means you have less time to invest in each child. Not to say people can’t have multiple very successful children but it gets harder each one you have and there is less money to invest for every child you have unless you are rich. I would bet if you tracked average educational achievements you would see a correlation between lower number of children and higher outcomes. (Remember on average - not talking about people’s specific families)
so I don’t think the immigration argument flies in this situation.