I have been instructed to recruit a successor to my existing job, I also understand that my successor role and possible candidate is being discussed at board level, is this constructive dismissal? I have held my job successfully for 10 years

Do you have an employment agreement? Are you a member of a Union?

(I think this is a legal question and should be moved.)

I would take it as so and I would also be asking what happens to me after I have trained this successor.

Been there, done that. It is possible that your employer has something better in line for you. Let's hope so.

Good luck.