"it is unreasonable to expect young people with ADHD or other similar ND disorders / mental health challenges to work, even if they have qualifications and therefore they must be financially supported by the state"
No, it's not unreasonable. The vast majority of ND folk are totally capable of work, and actively want to - they just need some accommodations from their employers to do so, and sometimes support in getting an appropriate job. Support from the state would be necessary, sure, but only for the ND individuals who are demonstrably unable to work. This will require investment in the assessment process, given the highly-individual nature of the difficulties ND people face, but it can certainly be done.
As for the original question...it's simple: borrow to invest, with a good long-term plan for supporting that investment so that it generates a higher tax yield to cover the costs of the extra borrowing.
That's the only way this country's ever going to get better.
Critically, though, this needs consistent governance and will take a lot longer than the four years left of the current term, rather than what will actually happen - which is to say that the UK electorate will follow the drama and elect an entirely different government in four years' time, with an entirely different plan (regardless of whether it's an improvement, because "different" is enough), and the whole thing will be scotched. Rinse and repeat, and a recovery which could've taken as little as 10 years will take more like 30.
This has always been the way, no matter whether it's swung left or right.