Are you talking about in-car or hand-held? I have a hand-held one I use for hiking, but I'm also hopeless at directions. So when my old one died, I made sure to get one that could a) do topographics, b) do turn-by-turn directions but c) could easily be upgraded for the UK.
Most of them will do all three, but the Garmins (the ones I'm most familiar with) made you fork over $300 for maps of all of Europe, when I only need the SE corner of England. Then they came out with ones that allow you to plug in new maps on cards, and they sell a $95 card that covers exactly the area I need. I got the Garmin Colorado, plus City Navigator (I already owned the US topo). I'll buy the topo card shortly.
If you're talking about in-car, however, I don't know jack
Incidentally, if you wondered why the UK lagged in GPS and online mapping services, it's because the Crown owns the copyright to the Ordnance Survey maps, and buying licenses ain't cheap. Mapquest and Google and others hesitated before they bought in.