Usually when I fly into an airport I put my last waypoint into the flightplan and then let ATC decide what STAR i should fly and they give me the STAR clearance, since different STARs usually lead to different runways.

But for example, in the UK, the same STAR can lead to the same runway. So, if you write the STAR you'll be flying already in your flightplan, do you still need a specific clearance upon arrival to fly it? And if so, what if you do not enter you're STAR, will they specificlly clear you on to a STAR then?

The assumption is that you fly the published STAR that is attached to the end of your flightplan. If radio silence continues at the end of the STAR then you fly the published route to the landing runway.

 

For example if you were flying into London Heathrow from the South and the landing runway was 27L then you would fly the STAR from your last waypoint until BIG. And thereafter follow the published altitude, speed and heading restrictions until touchdown. In other words standard radio failure procedure.