I'm a single adopter. I decided to adopt in my late thirties and at the time I was earning a little less than you, but then things cost a little less then too. I found the year of adoption leave fairly easy on my finances as much of the year was full pay or holiday pay (also full pay) and my outgoings were quite low. I did have a period of statutory adoption pay and no pay, but I'd budgeted for it, and it was fine. The part I really hadn't looked ahead to until it was almost upon me was returning to work when my son was two. I was completely naïve regarding the cost of nursery, plus there was the cost of commuting, buying lunch at work, and I was having to provide for both of us on 50% of my salary because I went down to 50% hours. I probably could have claimed universal credit, but didn't realise it at the time, so just tried to live within my means and dipped into savings. Things certainly improved a lot when he started school as the only childcare I pay now is a childminder for an hour in the morning. Happily, this also coincided with a promotion and no more commuting (thank you covid), which gives me two extra hours in my day meaning I now work 80% FTE, and I've saved the money I would have spent on trains. Life does feel comfortable now.
So, I would say that 55k as a salary is fine to adopt, but you do need to look at the bigger picture. How much money do you have left at the end of each month? Are you saving? If not, then could you (as you certainly should)? What are your outgoings? If you are paying a substantial part of your salary on your mortgage then downsizing, as you suggested, might well be a good thing to do. And then you need to consider what life will look like as an adoptive parent. Are you really going to be on 55k or will you reduce your hours? As a teacher, it's great that you'll have school holidays, but in other ways it isn't the most flexible profession in terms of hours, managing your child's school run, taking your child to appointments, etc. Of course teachers often do have children, but how many teachers who are single parents of a small child (possibly with additional needs) work full time hours? You might think you could rely on wrap around childcare and you might be able to, but that really depends on the child. Not all (adopted) children can handle it. And as someone mentioned upthread, depending on the needs of the child, which are not necessarily apparent on placement, you need to be prepared for the fact that you might need to give up work.
If you really want to adopt, you will just have to make it work like many have done before you. Good luck in whatever you decide.