Nothing in this Act shall authorise the execution of any works whether of construction, demolition or alteration on, over or under tidal lands below high-water mark of ordinary spring tides, except with the consent of any persons whose consent would have been required if this Act had not been passed, and except in accordance with such plans and sections and subject to such restrictions and conditions as, previous to such works being commenced, have been approved by the Board of Trade in writing under the hand of one of the secretaries or assistant secretaries of the Board of Trade.