If you declare an emergency then YOU are flying the plane - not ATC! Get everything under control and THEN tell ATC what you are going to do...!
I don't understand these young ones that seem to think absoloutely everything they do in an emergency has to be confirmed by an ATCO before they actually do it. I've sat in a simulator session where one wanted to ask ATC if he could cut the fuel to the #2 engine after a simulated birdstrike!!!
Who cares if he didn't "technically" declare an emergency? He says 'we are declaring an emergency' - if it wasn't standard R/T then he can get moaned at afterwards. Not anyone's job to question his judgement. He has an emergency situation then he has top priority.