Callouts
Any time you go underground you should leave a callout with a trusted party. Generally it's a good idea to leave it with another caver as they will know what to do with it. You should tell them where you're going, who's in your group, any medical conditions, and what time you'll be back by. Their responsibility is to give this information to cave rescue if you don't return on time.
Calling cave rescue
If a party is overdue, don't wait to call cave rescue. To reach cave rescue, you should call 999, and ask for the police, and then for cave rescue. When you call 999, your call will go to a call center, which could be anywhere in the country. If you try and ask them for cave rescue they likely will have no idea what to do. Your local police force should be familiar with the local cave rescue team and will be able to handle your request.