I bought a NavMan 510 last year (from Halfords for 400 pounds). The same unit is now on sale from Halfords for 180ish. Its basically a dedicated PDA - and it has been very useful.
I bought it for my 18 year old daughter to use (as she cannot read a road map!!!). My son and I used it to navigate from Oxfordshire to the GlenGoyne distillery near Glascow, and then onto Edinburgh (and Glen Kinchie distillery and to see Concorde) - without ever having to use a road map ourselves.
The price of these units keep dropping, so wait another 3 months and a new unit might only cost 100 pounds!