Airline Credit Cards

I'm thinking of getting an airline credit card as I like flying to the
States and I probably run up £1,000 a month or so on my (non-airline) card
at the moment. I was thinking of getting a BMI mastercard as they offer a
free return flight to any european destination when you sign up, and they
offer 1.5 miles for every pound spent, and double that on airline flights.
By their tables (and you always have to be somewhat wary of these things)
you need to spend £30,000 on non-airline purchases to get a return flight to
Chicago. Does anyone know how this compares to other UK cards? I'm after one
without an annual fee, though the interest rates aren't a consideration as I
always pay my cards off in full each month. Any advice welcome.

American Express offers both Charge and Credit cards in a multitude of
variations. Both can be had with the AMEX Membership Rewards or with
specific airline programs.