As long as US legislation is not involved, and US taxpayer money is not involved, then I think most private agreements between US Charities and willing British citizens is probably none of my business.
It sounds like a good idea. These addicts can’t take care of themselves, much less kids.
However, paying them to be permanently sterilized is plain wrong. Addicts are controlled by the drugs; they no longer have free will, and are therefore incapable of giving informed consent. Paying an addict, who is jonesing for a fix, to get sterilized; is nearly the same as a forced sterilization — without even the benefit of legal due process. Forced sterilizations were a hallmark of the eugenics movement (modern offshoot — Planned Parenthood) — and were rightly outlawed.
Many addicts do overcome their addictions (perhaps not a majority — but, many nevertheless). If and when they have their addiction under control, there is no reason they should not have the right to have children. In fact, the possibility of having children is a major incentive for some of them to get straight.