www.airline-network.co.uk has flights from both London and Manchester to Dallas at that time of year for £201 return (I put in 7th-14th Feb, but it's probably the same for other slight variations). There are loads of flights for very slightly more, if you leave it late and those get booked up. I used Airline Network this year for flights to Heli-Expo in Florida, and later to Los Angeles and Moscow; they're very helpful, and will find accommodation and car hire if you want it. I tend to check the flights on the web, then phone them, so you can get info like that from a Real Person. If you have a choice, American Airlines have loads of leg room in economy; don't know about the others.
Whirly-Girls, of which I'm a member, had reduced price tickets for Heli-Expo last year, but I don't know if I can get extra ones. If I can I'll let everyone know.
I'm trying to work out how to get to Dallas for Heli-Expo and the Whirly-Girls annual "hovering", then do some fixed wing flying (Whirly hides her head in shame) with a friend in Florida for a few days. Do any of you across the pond know how much it would cost and how long it would take to fly from Dallas to anywhere in Florida? Or would driving it be remotely sensible. Or is the distance/cost too much for it to be sensible to do it all in one trip anyway?