For a long time BA has offered advance buy-ups to Club Europe, and on some international routes to World Traveller Plus, for nominal amounts. The Club Europe cost is usually around £69, but until now has only been offered on flights from Heathrow. The price quoted includes the increased taxes due for flying in a premium cabin.
Today, it appears, flights out of London City Airport, can likewise be upgraded. My offer – £69 for a flight from City to Amsterdam next month.