Malory seems to mention Soleise only once, listing it with Denmark, Ireland, the Vale, and the Isle of Longtains. The inclusion of Denmark and Ireland suggests that the other three were also outside the larger British Isle. The kings of these five countries united to war against Arthur, who defeated and killed them beside the Humber River.
The Isle of Man is enough outside Great Britain to count as "foreign", and is in a good position for alliance with Ireland; hence, it might make a good Soleise.
Five Kings | The Legend of King Arthur
Sarras | The Legend of King Arthur