Honestly, for the amount of time required and commitment for doing the Duke of Edinburgh, it might not be worth it. It kind of depends what activities you are currently doing right now. Like Heathrow said, air cadets and other aviation related stuff would be a big bonus I'd say. I was an air cadet and got my glider and private pilot's license through there on scholarship. it's quite a good program! we had the option of doing the Duke of Ed program but hardly any cadets I knew did it. To me it wasn't worth it and I don't think, at least in Canada, it's that well known and you probably won't get the credit you deserve for it.
If you really want to get into ATC there's nothing really better than actually knowing regulations and information about being a controller. Having a pilot background is a big bonus I'd say as you will be able to relate to pilots and know what they can and can't do. It will help you to make sure you don't give a clearance to an aircraft that he's evidently going to refuse. It also gives you some basic information and the air laws and who has right of way, etc. The more you know about that I'm sure you can bring that up in your interview. Go and get a good on being a controller. You can probably google an ATC course study manual, buy it and read it. Maybe even study for an exam and write an ATC exam and bring that in to the interview or include it with your application. If you get a good score on that and you haven't even been accepted, I'd let you in the course if I was program director!