I know that Carnival corporation companies have some strange rules and policies concerning UK bookings as I have several clients there. As with alot of our European clients, sometimes it's better for them to book with a U.S. agent and sometimes it's better to book with a local agent over there.
Honestly, I've never heard about this situation with singles. Of course, all my clients over there are couples or families, so I guess that's expected.
But this is really strange. They'll have specials over here whereby it's only offered to seniors, residents, military, etc, but when these super saver or early save specials are offered, they don't differentiate between singles and couples. So I can definitely see why singles are playing the game to get a better rate over there. It would be interesting to compare the different rates over there to what they are over here to see which way would benefit a single more. But as you mentioned, you'd have to make sure everything is the same (parking, onboard credit, etc).
What irritates agents over here is that occasionally they'll offer specials over there we can't get. We were once on a Princess cruise in the So. Carib (and by the way, U.S. agents cannot sell Princess to anyone outside of the U.S., Canada, or Mexico), and they made an announcement that 55% of the passengers on that sailing were from the UK. We got to talking to a UK travel agent and they were telling us about the special offered by Princess that gave them two 7-night B2B cruises PLUS airfare for less than U.S. agents could sell one 7-night cruise for!! Are you kidding me?
So I guess I shouldn't be too surprised about these specials you're talking about. They do seem to do some strange things over there.