[1] The rationale is that clients ought to be able to communicate freely with their lawyers, in order to facilitate the proper functioning of the legal system.
In the UK, "mediation privilege" is generally protected, although in the case of Ruttle Plant Hire v DEFRA (2007), an action brought to seek to set aside a settlement agreement on the grounds that it was entered into under economic duress, there was a call for the mediator to give evidence on her recollection of the mediation process.
[2] In the United Kingdom, the Rehabilitation of Offenders Act 1974 provides that evidence relating to spent convictions (those in respect of which the Act says the convicted person is rehabilitated, generally older and less serious ones) is inadmissible and provides privilege against answering questions relating to such convictions; although some exceptions apply, in particular in criminal proceedings.
[3] The effect of the privilege is usually a right on the part of a party or witness to a case, allowing them to refuse to produce evidence in the form of documents or testimony from the person entitled to the privilege.
Thus, if a wife wishes to testify against her husband, she may do so even if he opposes this testimony; however, the wife has the privilege of refusing to testify even if the husband wishes her to do so.