Amazon.co.uk have some great discounts on Sat Nav units at the moment.
My favourite is the TomTom XL v2, with Western Europe maps. Big screen and all the maps for Western Europe.
For complete Europe maps the best unit is the Garmin Nuvi 255W.
But, if you are truely intrepid explorer and require worldwide maps, then the TomTom GO 940 is the one for you. Yes, worldwide maps!