Even if we can increase the quantity of migration, if we aren't able to compete for the highest skilled migrants or keep our own young people in the country by offering them jobs as highly paid as other english-speaking places (e.g. the US, Australia, etc) with lowish taxes that allow them to build wealth, we're very likely to face serious economic problems supporting the public services we have through the ongoing demographic transition.

That is true. For the time being the UK is still able to sidestep this because there's a supply of well educated English speakers in an increasing number of African countries for whom the UK remains attractive (e.g. the UK is significantly increasing the cost of educating nurses for a lot of third world countries because the UK fails to educate enough of its own), but it will be harder and harder as competition for immigrants heats up.

Why isn’t the solution raising native English birth rates?

If you know a reliable way of doing so without imposing poverty or massive authoritarianism there are dozens of governments which would pay vast amounts of money for your expertise.

I don’t think that’s true, but if any actually are, I’m happy to help for a moderate fee plus costs.

What I see though are governments which oppress their people to the point of illness, the way abused animals stop procreating — and then rather than confront that behavior in themselves, buy replacements they subsequently abuse the same way.

Not a single country has found a working method, despite attempts at extending welfare, and extensive financial benefits.

But ignoring that, the end result is the same whether people can't or won't figure out how to do it: immigration is an increasing necessity, and as more and more countries tip below replacement it will lead to increasingly desperate measures to compete for it.

With respect to oppression, the two most consistent factors correlating with falling birth rates is higher education and increased wealth.

Imported labour is cheaper.

I'll be cynic, but basically this:

A manual laborer start to get productive at 16, and stop after 50, depending on his form.

So basically, you avoid paying for his education, he is productive right away, and with some luck, he will retire in his country (or the state may deport him) and die there.

Not saying it's good, but that's the way the world work right now.

You would be surprised, but that was the plan in 50s and 70s, when we got a people from Pakistan and South East Asia to come to work in the factories. They did not go back and settled here. The actual result is that they stay here and bring their families and contribute. Which is great, in my opinion.

"The UK would face economic collapse without significant immigration."

Demographics of our country guarantee a demographic collapse. Not as bad as say Japan or Germany, but enough of an impact to maintain wage inflation. The level of immigration that would be required to reverse that is currently not acceptable to our electorate (in the main). Which is unfortunate.