That graph is per-student spend, so the rise is not due to immigration. Education spending has risen per student in real terms (ie even allowing for inflation).
Yes, it's a choice. GDP tanked after the 2008 crisis, and Blair kept increasing the education spending. We all saw it as an investmentvin the future.
BUT actually, Tony Blair's huge cash injection in the early 2000s - to create a more-educated workforce - clearly hasn't actually increased our GDP 20 years later.
Of course, education is about citizen wellbeing as well as an investment for the future - it's fine to say we want to prioritise it over other government spending. BUT given the numerical evidence, we can't pretend that Blair's spending was an investment in GDP growth. We can still choose that spending, but you can't keep increasing it with an airy 'we'll get the money back later'. We won't. So to be sustainable, it has to fit into the GDP we have.
Here's an interesting graph of government spending across each department as a proportion of GDP (https://www.icaew.com/insights/insights-specials/the-future-of-tax-and-public-spending/graphic-70-years-of-public-spending)
Look at the trajectory of Blair's cash injections - in various departments - between 2003 and 2010 as a proportion of GDP!!
Choice is right. Our GDP has not risen. You want more Education spending? What do you want to cut to balance that?
("other people's standard of living, by increasing tax take" isn't an option. Not sustainably. )