Might I suggest that you have cause and effect muddled? Please keep in mind "Hate the game, not the player". Please forgive me for some heinous generalisations, but on balance everything below is correct for the average UK citizen.
Full disclaimer: I'm not a teacher, but I know several very well. I went to a private fee-paying school. I grew up in a Conservative household with right-wing, capitalistic views. At the time I thought Maggie Thatcher was awesome. Over the last 15 years or so I have become utterly disgusted with the Tory Party: the constant lies, the obvious skulduggery, the financial theft from the general public and the horrendous disregard for the future.
The system of education in the UK would appear to have degraded over the last 50-60 years or so despite (actually please read "specifically because of") constant "updating & upgrading" by our politicians.
In most countries, but specifically the UK, the posh/ rich/ elite are in control and they are very protective of themselves and their own. They have a habit of making rules/ regulations/ laws/ policies which on the face of it seem fair but have a nasty habit of handing them ever more power and wealth.
Key to this is education.
If you have the money you can ensure that your kids get a proper, rounded, well-balanced and thorough education by paying for them to go to a private school.
If you don't have the money then your are stuck sending them to the local state operated school.
That should be fine if the funding was correct, if the facilities were adequate, the number of properly qualified teachers per pupil was sensible, and the requirements for the contents of the curriculum were sensible.
However, by a series of what appear to be very sinisterly calculated moves, the Conservative Party, over a period of decades has torn our national institutions to shreds by continuously cutting funding and meddling with policies.
Every few years new policies are introduced that effectively dumb down the education provided to the average citizen in the UK.
The question is "why?". The answer is simple: Control. Control allows them to keep power & make money.
If the general population are too poor, too ill-educated, too busy, too glued to the TV/ smartphone and too stressed to think critically and question their overlords, then the overlords maintain and grow their political and financial power without having to fight for it. They are simply handed it by a population that has been hypnotised into being compliant mushrooms. The worst thing they have to deal with every now and again is a strike, which they blame on everyone else but themselves.
Indentured servitude with an invisible cage is still just slavery. Our population is running hard to stand still and the politicians are constantly blaming "THEM!" (pick whoever/ whatever you like to fit in there - it's the way that right-wing politics actually works with knee-jerk dog-whistle statements).
Worst of all, we have been persuaded that this is all in our own best interests!!!
Meanwhile, the posh/ rich/ elite send their kids to have a proper education so that the cycle can repeat all over again.
Blaming the unions is not fair. Teaching is an honourable profession and one of the most important ones. Teachers should be treated with respect and paid properly. The UK government has underpaid teachers for decades. They **DESERVE** a proper pay rise. As do nurses, doctors, the police, firefighters, and most other public service employees in the UK.
I would hope that in all countries, not just the UK, getting a good education isn't considered a privilege for the few but should be considered a basic right for all.
Why?
Because we are reliant on the next generation to look after us when we are old and frail. The last thing we need is a country filled with people who cannot make simple but sane, rational decisions for themselves. We desperately need a well educated population capable of critical thinking on an individual basis. We do not need some kind of hybrid meritocracy/ kleptocracy where the top 1% allow the rest of us to serve at their feet.
How to make the change? Educate our children!!!