What you're proposing sounds like a bit of a bodge - think about whether you would want to see this in a house you were looking to buy.
Personally I would either do it 'properly', wired into the lighting circuit with a proper light switch and all the connections accessible as required by the regs. If the loft space is ordinarilly inaccessible and you can't fish the cable through, you may need to access it some other way, such as making a hole in the ceiling, which will involve more work.
The easier option would be to use some kind of plug-in lamp.
Adam, March 2010