I would fund hundreds of new schools/convert current schools to include a special needs wing in addition to normal state school. This would mean all the kids struggling with mainstream but not disabled enough for special school could get the support they need and switch between the wings as needed. It would stop the struggling kids disrupting classes due to distress, mitigate school refusers due to less pressure and would include things like support with ehcp and diagnosis.
I would pour money into developing more efficient energy storage techniques, better solar panels and turbines, carbon scrubbers to clean the air.
I would pour money into some of the fascinating projects in Africa and and south America that are reclaiming and regreening the desert.
I would buy a large island and make a self sufficient eco town. For my own security but also an experimental community where people would want to come and invent new technology.
I would fund several of the already existing programs that rescue women and girls from forced marriages or whom want to leave countries with Draconian anti-women laws.
I would change the curriculum to include a 'life skills' class that would include information on consent, red flag behaviour, legal information on marriage and property, wills etc. basics of running a car amount other stuff. It would be a daily 10 minute thing at form time or similar and be fun and interactive.
I would lobby the gov to sort out the child maintenence service to have more teeth more along the lines of the American model. I would set a weekly charge for an absentee father regardless of income (children aren't free). Maybe £50 per week and this amount accues over time. If the person wants to be exempt from this then they have to actively prove they are e.g. disabled. They can't become a sahp to second spouses child for example and get off paying because they choose to leave their job.