Thanks Lucinda, I managed to get the ketones down yesterday afternoon, so they let me come home yesterday evening with a prescription of ondansetron and prochlorperazine.
I slept 12 hours last night. Still exhausted and nauseous but at least home and hydrated.
Word of advice for everyone - find out what to do if your condition deteriorates before it does. I didn't think it would get that bad and had no idea who to contact.
The midwife on the ward said I could have phoned maternity assessment, told them my ketones and they would have asked me to come straight in (they only ask people to go to GP first for ketones reading, for those of us measuring ourselves we can just call them).
Instead I waited until "office hours", called my GP, waited, called them again (told they will call when they are free), waited called again saying I was getting worse and 7 hours after I first called my GP called back. He immediately made an emergency appointment to see him as I needed to go to hospital. Instead of phoning maternity hospital he phoned on call gynaecologist and wrote a letter and sent me to a&e. I spent an hour in a&e, nurse found me throwing up in toilets and decided to speed up my admission and sent me to the maternity assessment (where I should have been in first place). They got me on drip as quickly as possible, then got me onto ward, but was 12 hours after I sought medical advice.
So please everyone, if it's bad enough to need medication, it could get worse, so find out what you need to do before your ketones rise (and hope you won't need it).
It wasn't in my maternity notes or in the leaflet that says what to do if something goes wrong in pregnancy, my GP receptionist didn't know, my GP didn't know. Once I am better I will write to GP about this. I wish I had asked midwife at booking.