If you are flying on BA within western Europe & have a fully-paid economy ticket, you can upgrade to business for 16 pounds each way (about $27). This can be useful if you need more executive club points for qualification to the next level (ie blue, silver, gold), as you double the number of points you receive for the trip by doing this. It is also useful, if, like me, you miss your early morning flight to Cologne and have to fly to Dusseldorf instead - full fare economy doesn't allow route changes, while a 'Club Europe' ticket does.

As for Catman's question on another thread - a Club Europe ticket does give you lounge access, though the lounges aren't quite as pleasant as the Club World lounges.