I carry an eTrek (cost <100 pounds) which I plug into my PocketPC and run Memory map on. Total cost I'd say was < 500 pounds. It is excellent, I have downloaded all pertinent waypoints to the eTrek ( VORs, NDBs, Airfields) in case the PocketPC goes tits up. However the great thing about the Memory Map charts is that they are exact digital copies of the CAA charts, and they're 100% accurate. No database issues. You can also zoom in to a larger scale chart, so I have the 1:500000, 1:250000 charts, plus a few road maps, and also the airfield diagrams. The airfeild diagrams are brilliant. We landed at Edinburgh, in misty conditions, and it was great for taxying to the GA terminal, you could see your position in real time on the airfield. The charts cost around 50 quid per sector (ie. UK South, UK north and Scotland) and each sector comes with the 1:500000, 1:250000 and airfield diagrams.
Heres an example of using GPS as a safety device. We left Edinburgh cleared to GRICE or wherever it was, and almost immediately entered IMC. We were in a SE aircraft, and monitored our position on the GPS. Had we had an engine failure we would aim to ditch in the Forth via GPS. The cloudbase was very low, reaching up to around 4000'+. We didn't use the GPS for primary nav, but had we lost conventional nav or in an emergency we would have.
Its also quite nice when you get home to download the track from the PocketPC and analyse your flight on a chart of the UK (see how bad your navigating was
).....
EA