The crazy thing is that there is no overarching definition of an "overseas" player.
ECB would allow Jennings(on the back of a UK passport) and Chase(on the back of an EU passport) to play for Durham, and not be classed as "overseas".
In the ECB Premier leagues, they would both have played as "overseas". Jennings fulfilled the 4yr residency qualification and then "qualified" to play for England.
Michael Richardson, I believe, played, initially on a EU passport(German, through his mother) but now because of the time he has lived here and made his home here he now has a UK passport.
You cannot make this up !