On the basis that pictures are better than words, this might help.
http://www.diydata.com/planning/electric_light s/electric_lights.php#wiring
There is no difference fitting an external light and fitting a new light inside the house, in terms of wiring - you need to find a way of breaking into the existing downstairs lighting circuit at a ceiling rose.
This site also advises you about 'Part P' - the Building Control regulations that impact electrical wiring. Or some bright spark on AB is bound to say that you cannot do this type of electrical work without being a qualified electrician. For the scope of work you are proposing you can do it, but you do need to be competent and sure of what you are doing.