Well your 241 is different that what we English people are used to :P

Out Amex vouchers have the "outbound travel before xx/xx/xx" rule.

Yours seems different, however, it seems like you can have the departure until 15DEC, with the return until 31DEC.

You may want to check the T&Cs of your voucher on ba.com if it is electronic or somewhere in the letter/on the back of the voucher if it is paper voucher.