This is the Lightship Gannet, photo not by me. It was built in 1954 in Dartmouth and until the end of 2009 it stood off South Rock lighthouse in the Irish Sea off the county Down coast (bet the writing on the side gave it away!).
It has since been purchased by So long and thanks for all the fish, which, I suspect is not his real name. The lightship is now situated in the Medway where it is being converted.
Mr SLATFATF originally intended to buy a barge and convert it but decided that it would be much less work to buy a lightship. If you want to see what "much less work" looks like, there is a blog dedicated to its conversion. I wonder if there was a "much much less work" option?