> some sort of jelly fish monster, a lighthouse, Tom Baker

This must be Horror of Fang Rock, from 1977, written by Terrance Dicks under Robert Holmes as script editor. In 1974, when they held the opposite jobs, Dicks asked Holmes to give The Time Warrior a mediaeval setting, which he resented: so in good-natured revenge, for this script Holmes told Dicks he could write whatever he wanted, as long as it took place in a Victorian lighthouse. Since The Time Warrior has a Sontaran, Dicks decided his script had to have a creature who would be the opposite of a Sontaran, hence the jellyfish monster.