For what it’s worth, I’ve grossed £85k as a year 2 FO and received 11 day off payments, but that doesn’t mean I worked 11 of my off days, some duties are worth up to 3 day off payments for 1 day off. I’ve turned down a lot of flexi out of shear laziness and could have grossed an extra 5k easily. So £10k+ on flexi alone is certainly possible.
I saw my buddies payslip last week, he’ll hit 95k as a year 3 FO on short haul. Long haul FOs will earn more.