I too had the offer which arrived in the post on Monday.
Yesterday I received my BT Cellnet Select magazine and 1 or 2 other bits of promotional stuff from BT Cellnet. One of the promotions was actually the same promotion offered to RW members but without the KLM FD branding anywhere.
My guess is that BT Cellnet have approached various companies with this promotion to offer to their desired selection of their customer base.