Katy - I am so sad for you. My brother and sister in law in Australia had to terminate their pregnancy last year at 20 weeks for the same reasons as you, a condition that was incompatible with life.
She gave birth to the baby and they were allowed to name him, dress him and the midwife took photos. She had pain relief, although labour was slow and she felt uncomfortable.
As someone else said, I think 20 weeks is the point where babies are classed as "stillborn" and have to be registered, but I am sure you would be allowed to have photos if you wanted them and to see the baby. Although it was really hard for them at the time, I think seeing the baby really helped. My brother in law, who is a Dr, said you couldn't really see anything wrong with the baby - each case is different.
They had a ceremony with other parents of babies that didn't make it about a month later.
They underwent lots of genetic tests to explore why the cerebellar had not formed, but it was just one of those things.
She conceived again in August and she has had very close monitoring from the same hospital (2 weekly scans from 6 weeks) and offerred more in depth tests. She is due to give birth in early April and so far , everything looks good.
Don't feel judged or guilty.... this could have happened to any one of us. I really hope that you are able to mourn the loss of this baby and that when you feel ready, you go on to have a healthy baby.