Having flown to Florida with BA, Virgin, American and My Travel I can certainly recommend American Airlines. 31 inches of leg room makes a serious difference and the 2-3-2 layout of the 777 means the plane feels more spacious as well. American have made a point of not cramming seats into Coach and I think it works for a cheaper ticket than Virgin and BA.(Entertainment and Food are top notch as well, including the customer service).
I don't recommend going with the cheaper carriers such as MYT or Brittania as they cram you in with crap food and dirty planes. The normal eight hour flight seemed like about ten when I went with MYT.