It shall be lawful – means

In Julius v. Bishop of Oxford (1880) 5 A.C. 214 it was observed by Cairns, L.C., at pp. 222-223 that the words “it shall be lawful” conferred a faculty or power, and they did not of themselves do more than confer a faculty or power. But there may be something in the nature of the thing empowered to be done, something in the object for which it is to be done, something in the conditions under which it is to be done, something in the title of the persons for whose benefit the power is to be exercised, which may couple the power with a duty, and make it the duty of the person in whom the power is reposed to exercise that power when called upon to do so.”

Lord Blackburn observed in the same case at pp. 244-245 that the enabling words give a power which prima facie might be exercised or not, but if the object for which the power is conferred is for the purpose of effectuating a right there may be a duty cast upon the donee of the power to exercise it for the benefit of those who have that right when required on their behalf. Lord Penzance and Lord Selbone made similar observations at pp. 229 and 235.