When I lived in the UK I went through security clearance and they made me sign something waiving my right to dual nationality. It was ages ago and I'm not really sure what it is I signed. Is a dual nationality waiver an enforceable thing? Do I have to undo it in order to apply for dual nationality, or was it more of a "I pinky promise I won't try and apply for a passport while I work for you"? 

 @99019452
If it is the same SC stuff that I went through, you promised that you weren't dual nationality and that if you became one you'd lose your SC.

Your SC was probably sponsored by a department. If you don't work for them any more, you don't have SC.

So, no, you don't need to undo it.