Sir Thomas is a very controlling man, with a nasty snarl when things don't go the way he thinks they should. He is especially concerned that outsiders view the MP inhabitants favorably. Although we don't know what Lady Bertram was like when he married her (I will have to go back and reread the few hints we get in the first chapter), I think she is asking for his permission here - she doesn't want to incur his wrath if he doesn't think Fanny should go abroad. After living him with thirty years, she has learned her place.