Dear Constantine,
"Keiseguenon" is certainly incorrectly written. A prefix "Kei-" is not
located in the brittanic place-names. Most names there start with the prefix
"Ker-". Also the following part of the name sounds simplified very much and
at all not "brittanic". I think, one must look on the map for a similarly
sounding name.
At the Atlantic-coast of the peninsula Crozon in the Bay of Dinan ("Anse de
Dinan") gives a place, that is marked on the map as "Panoramic view" and is
called Kerziguenon. I think  Nordsieck has meant this place.
Best wishes,
Gert