I just booked my flight from IAD to LON on Expedia on two United flights. On the return flight, I purchased via BMI codeshare which was cheaper. It allowed me to pick seats earlier, but now that I completed the purchase, it says that United is not accepting specific seats and will not allow me to get specific seats until I check in for the flight.

Any suggestions on how to get around this. Should I call Expedia or try to go directly through United or BMI? Any advice is appreciated.