London Mini A-Z is a must have if you plan on visiting London. It provides you with a very detailed completely offline map of London. I used to live/work in London and traveled frequently there on business prior to living there and when I came back to NYC and i used to carry around the paper version of London A-Z and it was invaluable. With the iphone version, it's even better since the GPS will tell you where you are and that's what I use now. Visual IT also has Glasgow, Edinburgh, Manchester, and Birmingham Mini A-Z versions as well, not to mention their Tube programs for european cities that provides subway/tube information. I have that for both London and NYC now.
Keep in mind that if you do go from one region to another, i.e. NYC to London for example, the GPS might not be able to pinpoint your location prior to it timing out. Without getting into technicals, suffice it to say, the best thing to do once you hit a foreign city a far distance from where you last used your GPS, it will help tremendously if you connect to WIFI (wifi hotspot or hotel room) and then activate the GPS so that it will know your approximate location based on the router. Then outside, the GPS will know where to "look" for best satellite reception and you should be able to get a GPS fix then. Without doing this, you most likely won't get a GPS fix.
