Here's a look at lowest R/T fares between Orlando & London this past summer: as of July 7th -- $767; as of Aug. 31st -- $431 (both fares on Continental).
We'll be sailing on the Dream on Aug. 29th; our airfare is part of the package price. Our TA's invoice shows the cost of airfare (tax included) of $901 pp to include transport R/T London & Dover. We had considered booking our own airfare (and maybe saving a few hundred bucks), but have pretty much decided to pay the package price.
Package airfare prices can be tricky, and often include a reduced cruise fare in their calculation. So what you see is not always what you get. If you book your own air, you must make sure you get to London early enough to ensure being on time for the departure of your ship. That might mean leaving home a day earlier and a hotel expense in England.
Since we don't know what departure/arrival times and fares will be this August between Orlando & London, we'd rather not tempt fate in order to save what might not turn out to be very much money. If you wish to email me, I'd be glad to give you price information on our cruise.