First three months of life rules out anything in the UK except concurrent planning, which has the inherent risk of you losing the baby. Also, if you do concurrent planning you have to take the baby to contact sessions with his/her parents at least once a week, but it might be several times a week.
Otherwise, the US is the only other adoption option. You would get chosen by a pregnant woman as adoptive parents or called soon after a babies birth and take custody of the baby usually within a few days of birth. There are also extra considerations here - the trend in the US with infants is very much open adoption, at the very least a 'semi-open' (letters and photos etc), so are you comfortable with sending letters, photos, email updates to your babies birth mum throughout your childs childhood? Or maybe travelling back to the US once every couple of years and include a visit with birth mum? You would find it hard to find a pregnant woman willing to consider adoptive parents who will given her anything less than update letter and photographs of baby a few times a year. And obviously you will probably meet the mother and get to know her a bit before she gives birth. It's just a completely different 'adoption culture' than that which surrounds adopting from foster care so you need to be comfortable with that. Also obviously the mother has the right to change her mind after giving birth which is
The other issue inherent with either of these adoption options, is that you can't control anything that happens in the pregnancy. The reality is a good number of the children you could adopt (in both programs) will have family histories of MH difficulties/learning disabilities and/or exposure to alcohol/nicotine/other drugs in pregnancy, including in the last 3 months of pregnancy. If you're not open to a baby with this kind of health history, again you might find it difficult to adopt.
Surrogacy might be a better option, we have one or two mothers through surrogacy hanging around who can tell you more about that. Because the surrogate is having your baby, I assume she'd be conscious of keeping healthy in pregnancy and keeping the baby healthy, and if you care a lot about the babies health in the womb that might be a consideration for you