You may find that asking to pay for an upgrade at check in works OK in North America. I often do this when flying from YYZ (CAD350 to upgrade from WT to WT+) they send you to the ticket desk to make payment. The receipt has something along the lines of 'Special Upgrade Offer' and it is always the same price.

This may be BA promoting WT+ and you may have less luck getting into NCW but it is worth a try