Brythonek is an Essex-based Celtic conlang created by DJ Tyrer. It has been described in Brythoneg: A Celtic Language in Essex in Poetry Cornwall #37 and an article in A Breedon District Miscellany. A more detailed study is planned for a future volume from Buxton University Press.
Note on the name: Originally given as Brythoneg, this was revised to Brythonek and then Brydonek. It may be taken that the latter is the late form with Brythonek as the 'educated' form of the name and Brythoneg as a 'classical' or latterly Cymricising form of the name.