The English Channel (La Manche, literally "the Sleeve", in French), is a body of water that borders the Celtic and North Seas. It separates northern France from southern England.
In the home timeline, The English Channel was one of the only things that separated Great Britain from the European mainland, specifically France. This fact was the same across many alternates.
The Sleeve separated the Kingdom of Morgaf from the Hereditary Tyranny of Tartesh.
The Sleeve separated Dunlin from Ecnarf.