rmc1:
You'll be fine.
Opera would be the place where most folks would tend dress up (i.e Royal Opera House).
However it is up to you. If, for instance, you scored the Royal Box, then you might want to go black tie. But for the regular seats, you'll be fine. Heck, I wouldn't even bother with the tie (though it's a bit nippy and you may want the sweater or a jacket, just to keep warm).
Enjoy!