When I went to the US last year I flew with United. The return flight from Edinburgh to Heathrow were free as were 2 of the flights within the US. The other flights I had were £50 each.
I know that United are in a bit of financial trouble but on the occasions I flew with them they were full, were fine and all the flights were on time with no delays. The UK flights were on BMI.
Might be an idea to give them a call to try and get cheaper but it will mean having to go via LHR which is not very convenient.