It is not selfish in the slightest. I know MILs often get annoyed about things like this but you have such a real reason for doing it that if she even dares look less than happy about it she's being very unfair. Why not tell people you're going to get married and then go? No harm in that. And tell people you don't want anyone there! Only thing is, you'll need witnesses, and also it's tricky getting a marriage license in other countries- what many people do is have a quick registry office do in the UK and a blessing abroad, but guessing that's not what you were looking at doing?!