The Navicore kit should be available. Select UK and you will probably be given a drop down with cities, and just choose one near you. Or there will be online stores where you can buy from. Here is a brief overview of the navicore kit. I don't have it since it is hard to get in the states and it's a little expensive for a no so robust piece of gear.

If you install Maemo mapper you can download maps onto the N800 and use it there. Check out this forum thread for maemo mapper for installation instructions.

You don't need to buy the navicore kit, you can just purchase any bluetooth gps receiver and pair it with the N800 and use Maemo mapper for maps, it would work quite well...
I purchase a holux 240 and paired it with the N800 and it works well. It isn't as feature rich as the navicore, but with time and tweaks it might serve your purpose.

Hope this helps...Good luck,.