I am in a similar quandry. My flights are in WT (not sure the exact class) but BA Holidays have informed me that you cannot use MFU with them. The option is also greyed out in MMB.
I recently posted a similar question on another thread. Apparently if I pay to upgrade to WT+ (with BA Holidays) I can then MFU to CW (by contacting the BAEC) as apparently all WT+ classes are eligible for MFU.
I am a bit apprehensive about this as I don't want to get stuck in WT+ as I don't believe it represents VFM over WT.