The best deal by far is to credit your rental car miles to Southwest. You get a half credit per rental (though Alamo is currently offering 1 full credit!), which means you get a free flight in 32 rentals.

Most other companies give 50 miles, which means you need 500 rentals for a free flight, so the Southwest deal is 15.625 times better! (How's that for math, huh?)

The BA deal isn't bad, but you would still need 50 rentals for a free flight.

JP