You've got a bit of choice round these parts mate. Not the cheapest option for a Holiday it has to be said but, possibly the best one you'll ever take. A week would be better as it takes you a day to chill out, then you just watch the world franticly running round all around you as you mooch along at 4mph. Been on a few as my folks own a boat, can't reccomend it enough mate, just check out the boats in the marinas first to make sure they renting you a pup.
http://www.black-prince.com/bases.html
http://www.claymoore.co.uk/dayboat.htm
http://www.canalia.com/waterwaysdirectory/holscl.htm