Addicts are being offered up to £200 cash to be sterilised so they do not give birth to drug dependent children.The people funding this, Project Prevention, are doing so out of concern for the babies born to drug addicts but there is another aspect to the project.
There is probably a genetic basis to addiction so preventing it being passed on would be an effective means of reducing the problem in future generations but acknowledging the hereditary basis of negative traits is not considered acceptable, even though more and more human traits are found to be influenced much more by genes than the environment. So no one could propose it on that basis.
I'm not sure why pro-life groups like the Society for the Protection of Unborn Children oppose this though, it isn't as though they are aborting foetuses merely preventing conception. I don't see why it is any different from using contraception from their point of view. Surely even the broadest definition as to when a foetus becomes a child doesn't extend back before conception.