When you book an MFU there has to be availability in the original class of travel (W or T class) and in the Club reward bucket - U Class.
The lower price is probably the T fare and the higher one is the W fare - not sure why you are not getting the T fare for you upgrade if there is availability on a non-mfu flight - it should just go for the lowest fare.
You could phone and ask but will get charged an extra £15 if you book on the phone.