Depends how much you want to spend.
A non-aviation GPS (take your pick - lots available with broadly similar features) is useful. Up the scale a bit a Pilot III is very good. The Garmin 196 is better still but lots more cash of course. It is significantly better than the Pilot III because the screen is that much bigger and the screen updates much more quickly when you're scrolling around (not an issue when in flight - the III updates fast enough for that). I don't rate the 295 too highly because of the problems of seeing it in direct sunlight and the rate at which it consumes batteries (if you've not got a power outlet in your plane). The 196 has a decent yoke mount which works well in the planes I've tried it in.
None of these units do a great job on the ground, even the 196 and 295 that have a routing mode, though they do the job at a push. A PDA with a GPS will do much better, with more options and voice routing. If money is no object I'd recommend a 196 plus an iPAQ showing your CAA map. Then get road routing software for the iPAQ for use in the car (like TomTom). The 196 is a bit big and cumbersome to use just as a GPS feed for the iPAQ in the car, so throw another 250 quid at it and you can have a bluetooth GPS which is tiny and has no wires.
If you have more specific questions fire away.