Yep, I second that. Return flight LGW-TUL (via IAH) for me was £350 with ebookers.com. It cost $100 to change. Don't bother with single tickets or fully flexible tickets- it WILL work out more expensive!
It depends what you need to do out there in Florida as to how long to book for. The weather in January should be pretty good, so no problems there. Just ask the school how long they expect it to take you, add on 10% and there's your return date.
If the worst comes to the worst, and you end up without a changable ticket, you can fly
http://www.southwest.com/ from West Palm Beach to, say, Newark, and get a really cheap ticket to London out of there.