From what I've read, in most countries with strong levels of migration, the level of underemployment is almost always lower among migrants than the native born. I'd be pretty astounded if the UK was an exception.

Even just having reached adulthood and attained secondary-school level education makes you a capable worker that the country they migrate to has had to do nothing to support until that point. Any small amount of extra investment to ensure they learn the language and obtain employable-skills on top of that is virtually always going to pay off.