Oh, well, the thing is, usually house-elves associate more with a place than with a particular family or owner. I mean, the elves who left the Manor to follow your parents? That's actually rather rare. I mean, it happens, but--well, they must have decided they would rather give up your home than serve a blood traitor for a master. Or something. And remember Kreacher never left Grimmauld, either, even when he had the chance, because he felt too strong a responsibility to maintain the place. And then when Sirius took up residence, he had no choice but to obey Sirius, you know?

So I think it's really highly unlikely that the elves who are still at Buckingham would consider themselves in service to anyone other than the people at Buckingham, and specifically your aunt. That's if she didn't do anything to make sure of their loyalty, which if I were her, I'd have done right away. Like Sirius did with Kreacher, only probably with, I don't know, more threatening.

I don't know, it depends on what she's ordered them to do or not do. I wonder if Rachel's contact in the palace could find out what the terms of their enslavement are.