Originally Posted by
LondonElite
Not to be that guy, but it shouldn't matter what document you enter the UK (or any other EU country) with. As a citizen of the EU, you will have whatever work/volunteering rights in the destination EU country regardless of the nature of the document you used to enter.
Yes that's right, my thought was really about questioning on entry as I had in mind the unfortunate American poster who got turned away for saying she was going to volunteer at the beer festival.
But the replies above are absolutely right - my initial advice about the different queues was an inadvertent red herring as of course it doesn't really matter which desk OP uses - he/she can and should use the EU ID. I've certainly shown my British passport at the foreign desk before when it's looked a lot quicker and it's not a problem.