Ensuring the midwife doesn't leave you

Are the any circumstances where a midwife has to attend you and cannot leave (i.e. to do paperwork or help someone else)?
I know a midwife has to come if you have a home birth, but that doesn't help me as I will be giving birth at the hospital.
Is it true that they cannot leave you if you are in a birth pool?
Do they have to stay if you ask them to?
Do they have to stay if you don't have a partner with you?
Does anyone know what their rules are in the uk? I would feel much more relaxed to know that a midwife would be at my side throughout the birth (I understand they need to change shifts as long as I get someone to be with me)