For your question regarding hovering over polygons and displaying information, yes with Tableau Desktop 6.0 there are selectable polygon features that can be used to display additional attribute information. There are also tool-tips that can be displayed by hovering over rather than selecting the polygon, and these tool-tips are configurable based on your attribute fields. Below is Knowledge Base article on the polygon features available in Tableau Desktop 6.0:
Using Selectable Polygon Features in 6.0
Regarding your question on the county layers, in order to be selectable and configurable, you will need to bring in your own county polygon layer. If you are an ArcGIS user you can use the following Knowledge Base article to convert a UK county layer into a file format that Tableau can connect to directly:
Creating Tableau Polygons from ArcGIS Shapefiles:
If you do not have access to ArcGIS, there is also an open source option provided by own of our users which is included on the "maps, shapefiles and points" by Joe Mako:
There may be other users who have already created this file, so I will start a secondary thread to see if we can see what others may have already created that they are able to share in case you do not have access to the polygon boundary file.