Quote: Realisticly I would need to be able to survive on £800 a month, allowing for times with empty rooms etc. I would see income from work as a bonus. How does this sound as a budget?
Possible, but seriously spartan. If you can live on land for this amount, then you can live on water. You'll find many people, me included, who have retreated to a boat when times have got hard, so actually its cheaper.
What is more difficult is to keep the boat in good cruising order and face the expenses associated with regular cruising away from base. It means keeping entirely away from marinas during summer and holing up in a cheap mudberth during winter - you really do have to be somewhere that electricity is available. You'll become something of a water-gypsy, would you enjoy this lifestyle? Incidentally a yacht that can take the ground easily is an asset here.