There are quite strong constraints on privacy by councils and their committees and subcommittees. All formal meetings (and decisions can only be made at formal meetings) must be open to the public. Councillors who are not members of the relevant (sub)committee count as public for this purpose and are therefore entitled to attend. The public can be excluded if a specific resolution to this effect is proposed and agreed by vote. This would included councillors who are not members of the (sub)committee. The resolution to exclude must give a reason (in general terms) such as the need to avoid a breach of confidence or discussion of personal staffing matters. The public should not be excluded unless it is necessary.
In addition, all meetings must publish an agenda, which at least gives a clear outline of each matter to be decided. Decisions must be minuted (even in cases where the public has been excluded) and citizens (including councillors) have a statutory right to inspect minutes. It is never lawful to keep decisions secret.
There's not much that can be done to control what is said while a committee has excluded the public if it does not appear in the agenda or the minutes.