My number one choice would be BA, not only is it the cheapest, you get to fly more, and on different airplanes. Furthermore BA's service is a bit better than CO/SK and as BA uses T5 at LHR, the connection will be seamless.
If you want to go with the non stop flight I would go with CO. It is just not worth to pay that much extra to fly on SAS. The 757 is not that bad across the pond (have flown EWR-ARN on CO 757).