OP I have cared for women in your situation before and I've only ever seen them deliver naturally. Usually you will be given some medication into your vagina to help you dilate. As PP mentioned you don't need to dilate completely as an 18wk baby is tiny. If you choose not to see baby that is fine, the staff can wrap baby up and take baby away quickly. Also as another PP mentioned get ask for heartfelt to come in and take some photos. Even if you don't want to see them now, at some point you may want to see bub. If we have women in that situation we take the photos and put them in her medical chart so it's filed and that way if down the track the woman wants to see the photos we have them.
I can empathize with you not wanting to deliver baby vaginally as I can imagine it's quite an emotional time, but please think about the risks of having an operation. I am yet to meet a doctor who will do a caesarean type op in this situation as it is a major op and you'll be even more sore for days after. Maybe they would do a D&C, but to be honest I have only ever seen it done after a baby has delivered vaginally and the placenta hasn't come away.
I hope whatever happens happens quickly and you have good support from bereavement counsellors and midwives when you deliver your baby. This baby may not be meant for this world, but it doesn't mean that baby should be loved any less. It is your baby and you will remember him/her for the rest of your life. Words are never easy in times like this. Take care and please ask for help/support from your midwives if you need it. X