Hi
I've been answering you in very broad terms, as I've no idea of the existing circuits, the circuit protection, where you propose to run the cables, your ability to work with electricity etc.
I have to say, as I've already indicated, this work would come under the building regulations, and is notifiable.
To explain all you need to know to carry out this work is far beyond a forum. You really would be advised to get professional help on this part of your project. A registered electrician would carry out the required testing, and certify the work for you.
Please bear in mind that safety is vital, and a loft can be much more of a fire risk than elsewhere in the home.
There are one or two books available that deal with DIY electrics, have a read, I'm sure they would explain some of the regulations, and the correct procedures to enable you to take the best course of action.
As for your later posts, it's quite possible to run outside in conduit. The Marbo thingy - This circuit would need RCD protection anyway, and may involve replacing the consumer unit.