The point of socialized healthcare is that you pay even if you don't need it, so that other people can use it.

This, and say one day you need some serious surgery that costs 3 times what you paid in taxes. Socialized healthcare could cover this up to a certain point and nobody would come to you with a bill afterwards. To me it's the best approximation of "sharing health" a civilized country could come with, though some degree of empathy is required to fully understand its value.

Yes. However, only immigrants have to pay the £400 "NHS surcharge"; they are also paying for it through tax and national insurance. The government figured out that it could impose all manner of arbitrary charges on immigrants because they can't vote.

To be fair no one asked them to go to the UK. I can't vote in the country I live in because i am a foreigner, but it is my choice to be there, and I prefer it to my home country.

It's quite possible, even likely in fact, that someone asked them to go to the UK.

If you are talking about skilled individuals, then they probably are in decent paid jobs so £400 isn't a great deal to them.