Made this flight more than 25 times and BA has the best overall experience with price in mind.
We travelled to Fort Lauderdale last March to join the Pride with British Airways, one can be sure of good service and competative prices, you pay for what you get.
I would look at the Premimum economy class and use Virgin over BA. Remember that you can fly into Miami and transfer easily. I'm disabled and it's uncomfortable for me to fly for such a long distance.
When we fly with BA we fly club class so space is not an issue, if you join the BA executive club and have a BA AMEX card then you collect BA miles and can use them to book and upgrade flights. We are visiting friends in Dallas in a few weeks and have tickets in Club Class for £820 return! We also have Virgin miles but they have a different system in spending them on flights and most times it is cheaper to book with cash, they don't have the bokk/upgrade facility.