No. It is unlawful for a council to make a decision on any substantial matter that has not been clearly identified in the agenda. Plainly the agenda will sometimes need to be written with care to provide information on what is to be decided without breaching obligations of confidentiality.
If the public is to be excluded, a reason must be given, and a vote taken. The reason can be fairly general, such as the need to avoid breach of confidentiality or a requirement to discuss a legally sensitive matter, etc.