I'm not going to my friend's birthday party because of the dress code
What's the dress code for this party? It is something you cannot wear?
It's a traditional British royalty theme.
It's reasonable to want to skip a party that has a complicated dress code. It's not fair of your friend to expect you to comply with a British royalty theme for her birthday party, as that can be very expensive. However, if you don't go your friend will be disappointed and you may risk damaging the friendship.
I'm going to go to the party and try to wing it.
It is important that you do what makes you comfortable. I hope your behavior will be a nice compromise.