I work for Britannia as a 10 year FO and made a mere £4000 before tax this month. Mind you, I've been on leave for 27 days.
Lowest I've ever got was £864 AFTER tax.
FO flexi pay is about £357 before tax per day. Can't see that it would be possible for FOs to get £16500 without some sort of back-dated pay coming into it.