Did you realise you can nip across to Paris or Brussels, see an opera, and still be back in London in time for supper? Nearly all of their productions include at least one Sunday matinee which slots neatly into the Eurostar schedule.
Sunday shows in Paris start at 2 or 2.30, so even the new Siegfried (opening in March) ends in plenty of time to make the last train at 9.13.
Brussels is a little tighter. La Monnaie's matinees begin at 3, and the last train leaves at 8.29, so January's Parsifal is out. Most other shows are do-able though.
The train is of course not free, but some of the £69 cost is offset by lower seat prices for the opera - €180 max, and generally less.