I'm not sure that you have any idea of the cost of London moorings! If you are looking at a cheap way of building your dream, to own your own place and live in London, good luck to you. You might be able to build the accommodation and the hull for X amount of money, but the mooring will cost so much more, you'll wish you never bothered.
You only have to look at some of the posts on here with regards to the way the CaRT auction moorings have rocketed over the past few years. Boating is not what it was. We just left our mooring of 6 years, We paid CaRT £120 a month and the land owners (Well actually they lease the land from CaRT on a long term lease that's attached to the property deeds for around £270 a year!) They were paid £250 a month, Not a bad deal really, considering we had 2 full length boats. On handing in our notice, our landlords told us that they were just about to increase our rent to £795 a month!!!!!!!!! No shore line, no water, no rubbish or Elsan disposal, no address, Oh and we built the mooring, there was nothing there before we took it over.
Sorry for sounding negative, but boating has changed in recent years, certainly in the South, its becoming an expensive way of life that like a lot of things makes the rich richer and the poor poorer.
But good luck!
Oh and if you try Peter X's suggestion