They must be free of certain types of criminal conviction, such as fraud, theft, violence or any sexual offence.
As every local authority has its own licensing department and may have differing rules you should seek further guidance from you local taxi licensing department
50 % of taxi drivers in belfast have terrorist convictions included murder and firearms...i know a lot of them...becuase of the peace process they were granted them from council/doe /government as a means of making a living and leading a "normal " working life.