For the UK, a pretty definitive maps of submarine cables are held by KIS-ORCA (their cable awareness maps) at http://www.kis-orca.eu/downloads/. In their downloads section they have pdf maps for the whole of the UK coastline showing the landing points of submarine cables.
Here are some captures from April 2013:
They also have a superb interactive map at Interractive Map which is a map with switchable layers for power cables, oil pipelines, wind turbines, and so on.
A Google earth version is here:http://www.justmagic.com/GoogleMaps_SubmarineCables.html
Other sources are the TeleGeography map at http://www.submarinecablemap.com/
There is also Greg’s Cable map which although a useful source, appears to be work in progress. http://www.cablemap.info/
For industry updates, I recommend: http://www.subtelforum.com/articles/about-us/overview/. The excellent Submarine Cable Almanac is available for download, and is the best resource I have found for active cables including technical specifications and landing points, with maps.
Danish mapping application:Here (select Interactive Map)