For those spending time in London pre-cruise, Costco is offering a spring promotion on hotel packages that includes daily breakfast, $400 tour credit, private transfers and more with 4 nights starting at $930pp. If you call to book, they can split the transfers between the airport and cruise terminal in Southampton. Of course you can customize the package however you wish (additional nights, different hotels/room types, etc.) It may not be the most budget conscious option, but they are nice hotels in good locations, and did I mention private transfers?!? I think it is a good deal for what you get.
Oh, and if you price it out online first, they can see that when you call which makes the booking process go faster.