If you are a kid facing persecution and a high possibility of being murdered in your home country, coming to the UK and receiving an education here and going on to a career here is a massive utility gain, and if you go on to a successful and altruistic career it's an even bigger utility gain. The disutility of the kid coming here - maybe the teacher in the local state school has to split their attention between 31 pupils instead of the original 30 - is only a very small disutility.

Um, that style of logic doesn't work. You need to balance the (large but restricted to an individual) utility to the kid against the (small to each individual and spread out access many individuals) disutility to society. This is the kind of computation that's impossible to do intuitively (and probably impossible to do directly at all since we have no way to directly measure utility). It is, however, easy to see what the implications of a large scale population transfer are and to see that they are negative. You assert that there exists a threshold below which immigration is positive utility. However, you have no way to calculate it's value or show we are below it (or even show that it's not zero), without resorting to what looks like wishful thinking.