Well you have a valid excuse not to go on the holiday, because of your knee. Could you ring the people you're going with and say you can't go because your knee is swollen and painful, and that you can't even walk properly, and that if you went as you are now you're sure it would spoil your holiday and theirs, and that you're really very sorry, but maybe another time? Because it really won't help your knee at all trying to walk on it if it's hurting. I'm sure they won't mind if you don't go and they definitely won't hate you. If they don't like it then that's their own tough luck, they should be more considerate.