Of course it is entirely your choice if you wish to opt out. They may send letters but you don't have to go.
It may be worth considering your reasons though?
I had a smear at 25 - found it uncomfortable but OK. Then had my son at 27 and due to the trauma of the birth, I felt I couldn't face it.
So, although I wanted to go, I just couldn't and missed a couple. Recently (at 35), I saw a lovely GP about something totally unrelated and she asked why I hadn't had a smear.
I explained and she was so supportive and lovely about it. She said I could have a double appointment with a nurse, booked me with the most experienced nurse and said if needs be, I could just talk it through at the first appoiappointment and book another for the procedure.
Anyway, I went and the nurse was so lovely that I managed it in the appointment and it was fine. Result came back clear.
It is worth speaking to someone at your practice about it - they should be supportive.