There is really no such thing as a Quest 1, just a Quest. The Quest II is a different animal altogether despite outward appearance. The latter has it's own problems that I'll not go into here save to say best to avoid it.
I own a Quest, though last year I bought a StreetPilot 2720 off eBay so it's my reserve GPS (aside from the i3 and TomTom Navigator 7 on my phone
). A great unit; tough, compact and battery powered.
European Quests have 256MB of memory of which 243MB is available for loading mapping data into. The American Quest only has 128MB of which there's about 115MB free ISTR. The lower memory in the US model is due to the far lower road density over there. City Nav Europe is around the same total size as City Nav North America despite the latter having a far greater land area.
You can tell which model you have as the basemap is mentioned on the screen during boot-up, this should conform to the market region.
The whole of the UK will fit onto a European Quest with a little left over for a few mapping tiles of the French coast or wherever else you fancy.
As opposed to later models that can have onboard mapping for the whole of Europe going touring on a Quest entails loading mapping tiles (each country is made up of a number of tiles, each of which can be loaded independently) based around your planned route. Mapsource has a function just for this purpose. This is called your route corridor and, as Invicta Moto says, you can get a decent distance by doing this and only loading the tiles you require. Only downside is if you have a change of plan and decide to alter the route and run off the mapping you have available you'll be onto the basemap. This is routable but very basic having only main roads and motorways.
When loading mapping onto a Quest all previously installed mapping is overwritten so the available memory should be 243MB. When selecting mapping in Mapsource have the maps pane to the left visible and you'll see the total file size of the selected tiles down at the bottom.
I'm assuming that the mapping you have is unlocked to your specific Quest? If the unlock is a 'borrowed' one it would explain why it works on the PC but not on your GPS as unlock codes are tied to a/ the mapping product (i.e. City Nav Europe 2009) and b/ specifically your GPS by some internal serial number.
