The thing about ft funded v. pt unfunded is that you need to factor in loss of earnings. It would have cost me more to do a funded PhD because I would have missed out on three years of earnings.
Using OP's example of £25k over three years as funding, if I'm sacrificing a salary income of (let us say) £20k after tax each year in order to live on the stipend, I've given up £60k to do the funded PhD over three years, so there's a difference of £35k between salary and stipend. I'm also paying fees of £10k out of my earnings, but overall, I'm still better off by £25k to self-fund (not exact figures, as too lazy to work them out).
Of course, there is the prestige that comes with having secured funding, but against that, I can demonstrate what I've achieved in employment (which in my case, includes securing project funding, so that box is ticked).
This applies more to the humanities, where a pt PhD doesn't present the same logistical difficulties as one in science, and it presupposes that you're at a point in your career where your earnings are a lot higher (taking tax into account) than a PhD stipend. If you can shoot out of a ft PhD into a job that pays higher than the job held during a pt PhD, that would also affect the calculation.