I am not aware of any clearance for LABS, B47 yes, Vulcan no.
We had 3 relevant attacks, 2D, a climb to 27,000 feet for level release of Red Beard and airburst, 2E, a climb to level at 11,000 feet for a Yellow Sun ground burst and a 2H, a climb through 10,500 feet with release at angles between 12 and 15 degrees depending on Mark, engine fit and weapon.
Yellow Sun had a non-aerodynamic shape and would have been unsuitable. Red Beard could be delivered using Labs on Canberra, Buccanner and Scimitar.