Have a browse here
http://www.globalpositioningsystems....rated-gps.html You will usually find it cheaper to buy a package.
You will also have to take into account whether you need European mapping.
Using TomTom software as an example.
Some bundles come with GB Street level + Major Roads of Europe. If you later decide you require, for example, French Street level mapping, it would cost you £40 to download the map from TomTom and yet more if you wanted to add Spain or Belgium.
You could go for the Full package where you will get Street level mapping for most of Europe.
If you decide to buy a PDA with the intention of purchasing the maps later, make sure you get the FULL Software package, not just the Major Roads of Europe. If on the other hand you get a PDA with GB and Major Roads of Europe, you can add the Major Roads of Europe at a later date.
My own personal preference was a Mio 168 bundled with the Full TomTom Navigator 5 software. (Which can be updated to version 5.21 free from the TomTom site.)
One other advantage of the TomTom software is you can easily add Safety Cameras, or any other Points of Interest, and update them monthly from
http://www.pocketgpsworld.com/uksafetycameras.php
Good luck in your search.