I have not used Memory Map, so do not know if it can be used with c:geo. Addional offline maps can be installed though, for example these ones. I have the England and Wales map installed, and have just installed Belgium too. Using a combination of offline maps, like those, and a loaded PQ means you're setup for caching on a phone just like a dedicated GPSr and don't have to download a byte of data in the field.

The maps linked to above though are nowhere near as detailed as OS maps. But if anyone has any digital OS maps that are in the .map format, then they should be able to be used with c:geo.