The flight restrictions will likely be the reason
Looking on EF at the fares MAN-DXB r/t for april , the O1MAPGB1 fare has the flight restriction in rules that states
Code:
IF THE FARE COMPONENT INCLUDES TRAVEL BETWEEN UNITED
KINGDOM AND DXB
THEN THAT TRAVEL MUST BE ON
ONE OR MORE OF THE FOLLOWING
EK FLIGHT 0022
ANY EK FLIGHT OPERATED BY QF.
and the I1EAXGB1
Code:
IF THE FARE COMPONENT INCLUDES TRAVEL BETWEEN UNITED
KINGDOM AND DXB
THEN THAT TRAVEL MUST BE ON
ONE OR MORE OF THE FOLLOWING
EK FLIGHT 0022
EK FLIGHT 0020.
The next fare up ( IEEAXGB1 ) is valid on all EK flights and so would seem to be the 1st one that is valid for business class travel on EK18
As far as "flex" vs "saver" only the O class is saver, both the I fare that is EK22/20 only and the fare that allows all EK flights are flex fares