Although they are listed as one-way crossings, you might combine two Cunard transatlantic cruises for a round trip. The QM2 sails from New York to Southampton on 5/18/04 and returns on 6/5/04. That would give you twelve days in Great Britain. You might book a combination crossing with the QE2 for a different layover.
I was reading an interesting article a while ago, on freighter cruises. Its not a cruise as you or I know it, as you sail on board a container ship! Of course this would only be of interest if you don't mind the fact that there will only be a handful of other passengers (or even none), there will be no entertainment except a television, and you will eat the same food as the officers (not sure if that's good or not!). I think some companies run back and forth across the Atlantic all year, and I have heard the prices are cheaper. You can find out more on this website: http://members.aol.com/CruiseAZ/freighters.htm. Apart from that, the only way is via Cunard's QE2 or QM2 as the others have said. Some British cruise lines (eg P&O, Fred Olsen) regularly cruise from Southampton to the Caribbean and back but I dont' know how easy it would be for you to get to the Caribbean!