Depending on what view is taken of Galactus he can be either seen as a succubus, a galactic parasite who feeds on entire worlds, or he is a part of the Universe's natural order - his place in it is to weed out inferior worlds and thin out natural selection. A force that exists to test developed worlds and force them to rise to his challenge, Survival of the fittest.
There are numerous cosmic entities in the Doctor Who Universe, but nothing that is a direct analog of Galactus. So the closest candidates have to be entities who go after planets for their life-force, parasites like Axos and The Great Vampire from 'State of Decay', cosmic beings like Akhenaten and The Fendahl.
Wonder if there would be interest in a story that had gaklactus coming into His universe, and seeking out his world to devour, and the Dr offers himself to be His next herald, ala Norrin!