As someone who has had three generations living in my home, I support you for not wanting this. It's very, very hard. I would say that, unless you have grown up in a family that had multiple generations living together in harmony, avoid it.
I also felt a bit young to become a gran. But being a gran is not the same as being a parent. You have no rights and no responsibilities.
To those saying "sign up with a HA" fine, but this will not work unless the couple are living somewhere that is considered "overcrowded". Also, many HA properties have rent that is very expensive, these days. I don't think there are any council properties and even a HA property is not a "cheap" choice these days. I would recommend that they live with her Mother. It will mean your son moving out (and that's a loss) but no doubt he will visit? And try to stay on good terms with her parents.
Do what you are comfortable with. No Grandparent should have to change their life plans because their child chose to reproduce. It's not your child. It's your Grandchild, so the responsiblity is once removed from you.