I travel by road all over the UK and I bought a TomTom Go sat/nav unit last November. Now I would not be without it.
It comes complete with the UK map and the main roads of Europe but detailed maps of other countries have to be bought individually.
I use a couple of different vehicles and the unit can be transffered easily between them.
The current UK map is quite uptodate, M6 Toll etc is on there. The map does not have a full post code search facility but files can be download from the sites mentioned earlier and installed as Points of Interest.
The unit has saved me lots of time (and money not having to buy maps and street plans) and would recommend it to anyone
Hope this helps
CK
(ps I have no connection with any makers or suppliers, just a very satisfied user)