Avista:
I wasn't going to get into this discussion, but now I can't resist. A couple questions:
Did your agent / cruise line quote you a fare for your return flight? They should have.
Was it better than what you might find one your own for a one-way London-Orlando? Sometimes it is, sometimes it isn't.
A one-way across the Atlantic will be expensive - do consider the "bargain" of buying a round trip as duffysmom suggests.
When I have to fly somewhere, whether within the States or internationally, I start my research with Orbitz.com (to identify who flies my route and to see what prices are being offered). Then, I research the individual lines themselves. I try to stick with non-stops, where possible, certainly no more than one change.
Then, I go from there. I don't personally ever book with the cheapie on-line sites (Expedia, etc.), because I'm afraid of their harsh limitations and frequent failure to deliver.
Hope you can find a decent fare, because abandoning your transatlantic repositioning cruise just because of the expense of getting home doesn't, frankly, make any sense!
SWR