Remembering that you haven't remembered to send someone a birthday card in time. There is nothing you can do but look like an awful person who just sends a text and hope they forgive you.
Of course you can always send a card a bit later on, but they will know what you have done. If you use a first class stamp it will be obvious that you sent it on their actual birthday.
I wonder if there are rules for late birthday cards...If it is better to be a few days late and pretend it was just lost in the Royal Mail Postal system.