Reader Bohdan has kindly pointed out that if you fancy seeing any of De Nederlandse Opera's offerings this season, it's possible to fly to Amsterdam and sit in the stalls for less than a top price ticket at Covent Garden. With change left over for an ice cream.
Plentiful flights ( last return at 9.30pm-ish) make a day trip and matinee show a feasible proposition. Easyjet costs from around £50 if you book well in advance, and BA about £80. The heavily-subsidised tickets run from 62€ to 132€.