Try this thread as a primer on London Marriott hotels

http://www.flyertalk.com/forum/showthread.php?t=139154

I'd suggest hiring a driver to take you to Dover. A cab might not be comfortable, esp. with all the luggage you are likely to have; it's about an hour drive. I was in a group of eight last month heading out to Dover (Crystal Symphony) and we hired a 15 pax van - even with the extra space, once all of the luggage was loaded, it was pretty tight. I'd also suggest you stop in Canterbury on the way out and have lunch there and see the cathedral. I wasn't going on the cruise, so the driver was willing to take me back to London at no additional charge.