Hello. I'm an adoptive mum to two kids and will see if I can answer some practical questions for you. This must be a terribly difficult time for you and you need to take lots of time to think.
It's very unusual nowadays in the UK for babies to be given up or 'relinquished' (the legal term) for adoption - most adoptions, including my two, are of children who've been removed by social services from parents who couldn't care for them. That doesn't mean this isn't the right decision for you and your partner, but be aware that there isn't a well recognised route for you to go down.
In practical terms you would need to approach the social services department of your local authority and tell them you are considering relinquishing the baby when it is born. Both you and the local authority could make plans for adoption BUT you are not legally allowed to sign papers until six weeks after the baby is born. You can either take it home during that time, or it could be placed with foster parents. Up until the baby is six weeks old, you can change your mind at any time, even if the baby has been placed with foster or potential adoptive parents.
As you can see, if the choice in your mind is between adoption and raising the child yourself, you have a lot of time when you can explore the possibility of adoption, but can change your mind at any time,
If the baby was placed for adoption, you would probably have the opportunity to meet the adoptive parents. You might be able to discuss the kind of family you would like with social services although I doubt you would be allowed to 'choose' a family as mums apparently do in the US. The normal set up for contact is an exchange of letters and photos once a year between you and the adoptive parents. It's more unusual for there to be 'direct' contact (where you actually meet up with child and adoptive parents) but not impossible - you could ask for this. Most adoptive parents are encouraged to keep up contact with birth families but you have to understand that not all do and it's hard to force them. I hope that most adoptive parents would understand the value of keeping up contact with a caring birth mum and would do so. I certainly would. You probably wouldn't know where in the UK the baby was living.
If on the other hand you don't want contact, nobody would be legally able to force it on you. Bear in mind that the adoptive parents would be expected to tell the child they were adopted, and to give them information about you at least once they were 18. So it's likely there would be some contact in later life.
From an adoptive mums point of view, I can tell you that the process of being approved to adopt is rigorous, and most couples who get through do so because they want a child very, very badly. The adoptive parents I know are a very committed and loving lot and if you do go down this road I can assure you the baby will be very much wanted and loved. I adore my two and adopting them is the best thing I've ever done.
Your post does read slightly as if you are worrying about what you can give the baby practically. From my experience of adopting I can definitely say that the material stuff matters very, very little. Of course babies need the basics but the most important thing is that they are loved. If you don't feel ready to be a parent - which you will need more time to think about - that's a good reason. If you feel that an adoptive couple would give your baby a better material start in life than you, then I would say that's a bad reason, children don't see the world that way.
You do have some talking to your partner to do, and I hope you now see that you have some time to do this. I think that legally if he was on the birth certificate he would have parental responsibility and would also have to agree to the adoption. If you don't put him on the baby's birth certificate, only you have to make the decision - but if you've been together three years I think he deserves some part in a choice this big. Social services will also want to know his views when discussing adoption, I imagine.
I hope that's of some help. Please ask away if there are more practical things you want to know, and the very best of luck whatever you decide.