As far as I am aware, the only
PDA GPS moving map application which has adequate (and adequately accurate) UK/European coverage
and which supports flight/route planning is PocketFMS.
If you just want a GPS moving map that runs on a PDA, there are several other choices:
Memory Map (UK CAA charts only)
Oziexplorer (no aviation charts; you obtain bootleg ones, or scan in your own ones, works with any map whatsoever)
There are some other products that use dedicated hardware.
Then of course there are the dedicated aviation GPSs, but you asked about PDAs.
I don't like a PDA as the primary GPS. Too unreliable (Pocket/PC is a load of cr*p - imagine Windoze 3.0, 1990 vintage, level of reliability) and mechanically flimsy. It's OK as a casual backup. For one's main GPS, I would get the best dedicated aviation unit I could afford; perhaps a Garmin 296. Now of course 50 people will jump in, each with their own favourite
If you are willing to spend some money and move up so a "bigger PDA" then you can run other software on a Tablet PC. Something like a Motion Computing LS800 (do a google) will run a bunch of GPS moving map apps:
Navbox (also does flight planning; simply the best for European VFR)
Memory Map (very pretty display of the real CAA charts)
Jeppesen Flitemap (now discontinued but still "around"; can run the Jepp 1:500k VFR charts as a moving map).
The basic problem with the UK charts is that the CAA has decided to make money, by flogging their map data to Memory Map. The result is a set of very expensive electronic charts. All the while everybody moans about airspace infringements....
P2P (peer to peer) is a generic name for a network for sharing music, software, various other bootleg stuff...