There are several American products for PDAs which can take the NMEA serial data stream from a bog standard GPS receiver and which will give you a moving map display over your own chart scans.
You provide your own scans (TIFF probably) of whatever chart you like; it involves digitising the chart in 2 or 3 places to give the software the position reference on the scan. That is basically what MemoryMap is, functionally speaking, though it comes with scanned CAA charts.
I don't have URLs (don't need yet another GPS
) but you can post a question in rec.aviation.misc, rec.aviation.piloting, etc. The rec.aviation.* newsgroups are 99% American and somebody will be sure to make suggestions.
One could make a very nice GPS with e.g. the Toshiba E800 which has a full VGA 640x480 display. The quality is stunning... much better than any "aviation" GPS, at any price.
This approach is distinct from products such as Anywheremap which come with maps that tend to be poor outside the USA.
Do let me know how you get on!