I have back problems so most mainstream carriers (baby bjorn, tomy, shops own brands) are no good for me; nor is anything that goes onto one shoulder. I had a carrier called a kozy carrier with my youngest and it was great; he was a huge baby, about 20lb at 6 months and very long but he loved it and I could use it for hours up until he was about 8 months and not feel any strain at all. It was also suitable from birth.
https://kozycarrier.homestead.com/
Several UK sites also sell them. I still have mine and will be using it for this LO. I have heard napsack carriers and Joey slings are a good buy as well.