The answer lies in the "CARDS"
In BA we have a nifty peice of kit called CARD (don't ask me what it stands for). Before take off you send an ACARS message to the performance computer in head office specifying departure airfield/runway and all the met details together with estimated RTOW. A few minutes later you get the reply which shows optimum flap setting, assumed temperature for derate and Max take-off weight. The system appends the relevant performance notes including emergency turns and multiple runway requests are available. It also brews a nice cup of tea!
The performance assumptions are based on maximum engine derate for extended engine life and thus if any tailwind is present Flap 15 will be offered.........