Adopting after giving birth to a baby (UK)

Hi. My husband and I have a 20 month old and have decided not to have more children of our own. That said, we absolutely love the idea of adding another child to the family through adoption. We know the process can take a long time, but any advice on when to start the process? We were hoping if we would start soon, we would have done all the checks and maybe matched in two years? For what I have read, We would have the limitation of most adoption agencies or charities suggesting that the biological child be older and have a few years of age gap, meaning they would want us to adopt 1-2 year old. We wouldn't mind an older child, but just for what I understand and read on adoption. Any input or personal advice is welcome! We are also in Glasgow if that makes any difference.