Very sorry for the situation you find yourself in. Every child should be lucky enough to be born to their family & wanted enough to be raised. Nobody can tell you what to do of course, just making suggestions based upon what you have said here.
You do mention that if you continue the pregnancy you may have to take time off work if you are ordered to take bed rest again. Do you really want to go through all that again if you are just going to give away the child after the birth? How do you think your family would feel about that? Since your mother will be happy about another grandchild, perhaps there's some way she can assist your family at least temporarily if you need it, or maybe there are other friends or relatives that will come forward to help your family too. You said if you tried, you could manage. If you do feel you have enough room for one more in your family, then the best thing to do is raise your child with the others. You undoubtedly already have toys & clothes & other baby necessities from your other children. And the last child could share a room with your other child of that gender.
Govt. assistance is there for a reason. It's available to encourage families to stay together if they need it. Giving away the child will permanently effect not only that child but also your other children. They may worry that you might want to give them away too. Don't overlook the fact that will have a lasting effect on YOU as well! Please research the effects of closed adoption on mothers & children if you are still in doubt.
If you're asking us if bringing a child into the world just to purposely create an adoptee out of him/her, is going to make them happy under these circumstances, I would say that would not be realistic to expect that. Especially since you intend to subject them to the closed adoption experience. Whatever you do, do not expect thanks for that! I know I would not be happy about that. Every child deserves better than that or don't bother. You also asked our opinions, so yes, in this case, I think if you really don't want any more children, terminate the pregnancy early & then find a permanent method of birth control.
Yes, the one given away will always wonder why they were not good enough to keep when you were old enough, married, and had other children. I would not want to be the one to try to explain that one to my grown child. Also, I feel it's not very kind to subject an adoptee to a closed adoption. Those are usually reserved for cases where the child is in danger of being abused by the parents.
You say you want to do what is best. True, it is your body so your husband should be ok with you deciding on whether or not to terminate the pregnancy; however, once/if the child is here, he has equal legal say in relinquishing the child.
Final advice if you decide to continue the pregnancy - avoid all adoption agencies, adoption counselors, or solicitors online befriending you until after the birth. Make no decision until you have held your baby in your arms for at least a few weeks. Good luck,
reunited adult adoptee