So, you catch a 'thief'...

phoning 999 if it's not an emergency is

1. pointless as it would only be treated as a non emergency call anyway
   
2. abusing the system as a 'real' emergency may be waiting to get through
   
3. pointless again as all calls 999 or not are recorded
   
   

And you know all that for a fact?

Suppose I yanked someone's bag and ran off clutching a few hundred quid in folding beer tokens? Would that be a non-emergency crime?

Methinks I need a change of career...