Just to answer the question, as posed, you “opt out” simply by ignoring the reminder letters. However, I don’t believe that you can opt out of the letter themselves, as they are automatically generated and the GP could be found negligent, if at a later date you were found to have cervical cancer and you had not been offered the screening. You don’t NEED to do anything about the letters, bar recycle them. I’ve been told just to ignore them myself, because I don’t need to have smear tests anymore, but I still can’t opt out of receiving them.
I don’t know WHY you are choosing to opt out, although I can think of several possible reasons, neither is it any if my business tbh, but I would sincerely encourage you to think carefully about it. You have young children, as do many of us on here, and realistically most of us would do anything in the world to ensure to the best of our ability, that we are here to watch over them as they grow up.
Ive just read someone equating not having a smear to “crossing a road”, tbh, it’s more like crossing a road but wearing ear defenders and a blindfold to do so. We all risk assess the decisions we make, and are ultimately only answerable to ourselves, and to our children, not to people on public forums.