Give Rentalcars.com a call in Manchester( number on their website where you can initially price up). They will advise you there and then on one way charges, which are often hidden in the US and now free only, if IIRC, with Thrifty or maybe Alamo. You'll usually get a better price than BA.com and often the second driver is free. Tell them up front it's for two bookings; it might help the price!