IIRC BA have some cheap WTP fares that are actually an upgrade from an economy bucket, and therefore cannot be upgraded again from WTP to CW. I would check to make sure this is not what is happening. Again, book, phone and then cancel if you do not like the answer. All within 24 hours.

Or or just book through a TA who will not require the award space, just space in /I/. Makes it much easier and less stressful.

KF