Thousands of volunteers from around the world are in Ukraine.
But this was a base that NATO personnel visited. So our own
special forces 'Ex-special forces' have been inside Ukraine, which is basically what I suggested before, ie re training etc. It was also in the Guardian recently that Azov brigade had been incorporated in to Ukrainian forces. Ironically, this was in an article trying to undermine Russian claims that there were Nazis in Ukraine.
Regarding Ex-special forces. You never know whether they're volunteers / mercenaries or just not 'officially' in a role but in position with the full knowledge of whichever dept.