Check the prices on the AA website as well. They can still put you on a BA flight back to London across the pond, but have more connecting options on hand, too.
Oh, and we did Florida in July 2010 with the kids, and decided it was just too hot. Are you absolutely tied to the school holidays as if not you should make the most of it while you can!
There seems to be nothing in the fare between Economy and Economy+ for the journey back on the non-stop flights from Orlando, and
http://matrix.itasoftware.com is a good site for throwing back various combinations of flights for you in one go, like this one for £572 per adult.
London (LHR) to Washington (IAD) - Sun, Aug 19 Hide Details
British Airways 293 Dep: 5:15pm Arr: 8:20pm 8h 5m Boeing 777 Coach (O)
Orlando (MCO) to Miami (MIA) - Wed, Aug 29
American 2047 Dep: 4:25pm Arr: 5:30pm 1h 5m Boeing 757 Coach (O)
Layover in MIA 3h 10m
Miami (MIA) to London (LHR) - Wed, Aug 29
British Airways 208 Dep: 8:40pm Arr: 10:00am 8h 20m Boeing 747 Coach (O)