I have just had a quick look via my 550 and Garmin Express.
My 550's memory is very nearly full and when I go to update the mapping via Express, Full European and Western Europe selections are greyed out and I am unable to download them to the device....presumably because it realises there would be insufficient space on the Zumo's drive to take them.
It is a pity because other than the 2 mentioned above the only other selection which includes GB mapping is 'Northwest Europe Partial Region,' but other than GB it only additionally includes Eire, France, Spain and Portugal thereby leaving a whole large chunk of Europe off.
It appears you can have what Garmin say you can have and can't set up your own selection of individual countries.
With the limited space on the drive and bigger and bigger mapping I think it is a pity you are unable to select individual countries thereby making up your own mapset..............but it appears you can't (at least not via Express).
Does anyone know if I were to download say all of Europe onto a large memory card plus my music can I then listen to the music from the card whilst utilising the mapping on the card?