In the US market BA offers miles through three credit card programs: Chase BA VISA, Citi Diners Club/Carte Blanche, Merrill+ VISA, in addition to transfers from hotel programs. However, in the UK market BA has partnered exclusively with American Express, plus the same hotel programs. I think this reflects the difference between a carrier's home market, where BA will be the primary carrier for most flyers, and a foreign market in which they play a secondary role.