Only the latest B737NG's are CATIIIB capable. As half of U2's NG's are the older CATIIIA version, the whole fleet is operated to CATIIIA standard. And before you ask, no, it's not possible to upgrade a 3A to a 3B, and no, a crew cannot operate both types as the core actions in the event of an engine failure on approach in poor visibility are completely different. I think you'll find our Irish cousins also operate their shiny new NG's to CATIIIA for the same reasons.
