The basemaps are built into the unit and can not be deleted.
Purchasing EU maps such as Metroguide will provide street level mapping, and can be installed onto a microSD card. The newer units seem to come with 128Mb cards, which should fit the UK maps.
Reading these forums, it seems it is possible to convert / create maps which could be loaded into mapsource, but modifications to the registry are required.
I have installed UK Topo on my 60csx. Although it is not as good as OS maps, it least it shows the contours, and some roads. At least I have OS maps on my pda if required, as well as the cache data loaded into cachemate.
I looked at buying a 60csx from the states, but the postage and duty can be a killer. I managed to find an EU model on Ebay, and the slight premium was worth paying since I will be covered by Garmins UK warrenty, although based on 12 years of Garmin ownership, I should not need it.
If you are visiting the USA, then you can save more money, if you are prepared to wait til your next visit.
In the short term, if you have a pda with a gps (built in or bluetooth), then I suggest downloading and installing cachemate. The demo version is limited to 10 caches loaded at any time, but you have to 'navigate' to screen which will take you the cache. I have used it when I didn't have my Garmin with me. If you are a premium member and download the gpx data files, then you will also have the cache data for paperless caching. The full version of the software was £5.04 when I registered ($8 + tax).
Ivan