Should I accept invitation to be an external examiner for a PhD?
Since you do not have a PhD degree, you should check to see if the university's regulations will allow you to serve. The University of Nottingham's regulations state:


  However, it will be appropriate in some cases for Schools to nominate individuals from other suitable environments provided that those individuals are experts in the field of the student's topic of research and hold a professional status which is equivalent to a University Professor, Reader, Lecturer or teacher.


You should check if your experience allows you to serve as a reader. You wouldn't want to get involved and then find out it doesn't count for procedural reasons.

Otherwise, if you'd like to do it, there's no reason why you shouldn't!