Sure you can have designer decorations; have you seen the price of Christopher Radko ornaments? Especially the limited-edition ones.
I don't think the British royal family goes in for anything particularly spectacular, ornament-wise. Knowing the Queen's taste in things, she's probably still using the same ornaments that Mummie used back when she was Queen.
Since we already have a thread for Christmas decorations here, and since you're asking about the way the royals decorate for Christmas, I'm moving this thread to Royal Chit Chat.