Loomouth is location which occurs in both the Miss Marple and Poirot universes and may or may not refer to the same place.
In the Miss Marple universe Loomouth is a coastal town. In Nemesis, Miss Marple says that St. Mary Mead is halfway between Market Basing and Loomouth, which is about 12 miles from her village. At the same time St. Mary Mead is about 25 miles from London. If Market Basing is taken to be Basingstoke, then the distances work out about right, since Basingstoke is about 25 to 30 miles from the south coast. Loomouth could then be Southampton or Portsmouth. If Market Basing is based on Wallingford, this doesn't work out as it is considerably further to the north.
In the Poirot universe, Loomouth is the location of Crow's Nest, the home of Charles Cartwright in the novel Three Act Tragedy. The distances mentioned in Nemesis do not work at all as it is stated many times in the text that Loomouth here is far to the west of London in Cornwall. Like the Miss Marple Loomouth, this Loomouth is also a coastal town or village and has a harbour.