I've got an n800, so no GPS.

Quick-tips (since I'm at work too):

Grab a few maps automatically to find areas you want to zoom in to.
Menu -> Maps -> select Auto-Download
Then feel free to turn the auto-download off after you've got some base maps to start from.

To download specific geographical areas, centre the display on, for example, London - stay zoomed out so that the whole area fits into the screen.

Menu -> Maps -> Manage Maps
(There are options for downloading maps by area or along a route)
Select the Zoom tab and choose which zoom-levels you want to download. You probably won't need to go all of the way down to level 1 since at that level you don't get much more benefit, but there'll be a load more maps - I found around level 4 tends to be as far as I usually need to zoom.

Download the maps and you're done.