I was told it was first come first serve by my midwife and that when I was in Labour to give labour ward a call and explain you want to use the birthing pool, Once you're in labour and phoned them they will keep it for you as long as no 1 else has phoned before you
At Darlington Hospital you can give birth in the pool and have gas and air whilst you're in the pool, and if you wished your partner can get in the birthing pool with you. I didnt like the idea of that but i ended up having to have a c-section in the end so i never got there so im afraid thats all I know.
I do know you cant have any other forms of pain releif other than Gas and Air but you probably already know that
Every hospital may be different so i'd check with your midwife.
x