Lord Taylor of Gosforth, (starting at page 503) said,

So stated, the principle seems to conflict with the long established rule that a document protected by privilege continues to be protected so long as the privilege is not waived by the client: once privileged, always privileged. It also goes against the view that the privilege is the same whether the documents are sought for the purpose of civil or criminal proceedings, and whether by the prosecution or the defence, and that the refusal of the client to waive his privilege, for whatever reason, or for no reason, cannot be questioned or investigated by the court. I therefore turn briefly to the history of the privilege to see to what extent these traditional views are borne out by the authorities.

[He then reviewed the authorities]

…In Balabel v. Air India [1988] Ch. 317 the basic principle justifying legal professional privilege was again said to be that a client should be able to obtain legal advice in confidence.

The principle which runs through all these cases, and the many other cases which were cited, is that a man must be able to consult his lawyer in confidence, since otherwise he might hold back half the truth. The client must be sure that what he tells his lawyer in confidence will never be revealed without his consent. Legal professional privilege is thus much more than an ordinary rule of evidence, limited in its application to the facts of a particular case. It is a fundamental condition on which the administration of justice as a whole rests.

It is in the wider interests of all those hereafter who might otherwise be deterred from telling the whole truth to their solicitors. For this reason I am of the opinion that no exception should be allowed to the absolute nature of legal professional privilege, once established.

Edited for ease of reading.