http://www.bailii.org/uk/cases/UKSC/2016/47.html

One of the issues in this case was when will the law treat the authority of an agent as irrevocable. The general rule is that the authority may be revoked by the principal, even if it is agreed by their contract to be irrevocable. The revocation is effective to terminate the agent's authority, but may give rise to a claim for damages.

The main exception to this rule is where the agent has a relevant interest of his own in the exercise of his authority (and two conditions are satisfied: 1) the authority is agreed to be irrevocable; and 2) the authority was given to secure an interest of the agent). The Supreme Court held that the exception should not be confined solely to cases where the authority exists solely to secure the agent's financial interest: it can apply where the agent may be said to act both for himself and his principal.

Lord Sumption went on to note that "There are a number of special cases in which the authority of an agent has been held to be irrevocable on what appears to be a wider basis". These include the irrevocable authority conferred by a Lloyd's Name on his managing agent to underwrite (see Daly v Lime Street Underwriting [1987] and Society of Lloyd's v Leighs [1997]). Lord Sumption then commented that: "The result in these cases was undoubtedly convenient, but they do not lend themselves to analysis along the lines discussed above. Nothing that I have said should therefore be taken to refer to them."