The MBNA America MasterCard co-branded with Amtrak's Guest Rewards program is a fine tool.

You will automatically accrue 1:1 Gust Rewards points with every purchase dollar spent, but those points can be transferred easily and on-line to United, Midway, or, for you masochists out there, Continental.

Speaking of which, if you are accruing miles for an award flight to London, you are definitely better off accruing those miles is another airline's frequent flyer program, as actually getting awards particularly in BusinessFirst (international) via Continental OnePass is much more stingy and convoluted than the airline makes it out to be. Get ready for an uphill battle, especially if you don't have Platinum Elite status.