You are entitled to a lunch break after six hours of work, which means if you work six hours or less, you are not entitled to have a break. After 6 hours of work, you are entitled to at least 20 mins of uninterrupted break.
Some nurseries are happy to give one hour but not all of them. You always have to ask the nursery, what time can you take your lunch break and how long your lunch break should be? If they really need you, they will not give you more than 20 minutes. There's nothing we can do about that. You are there to help with the children in the room, so if you are needed they will not be able to spare you for longer than 20 mins.
https://www.gov.uk/rest-breaks-work