Not sure I get this – the Flight booking system is showing a seat plan now which indicates which extra legroom seats are available and invites a selection. How could this ‘work’ if there are different configurations and it is not yet known which configuration is flying the route on the day in question ??

Im saying this because i have checked in flights for Jet2.com and frequently had passengers who have prebooked there exit row seats months in advance only to have them changed in the days before their flight due to the aircraft been a different configuration.