Protest votes
It is well documented that incumbent governments fair worse at local and european elections. They are often used for protest votes that wouldn't be cast during a Westminster election.
From a comment from Andrew McEwan:
"Protest votes
Andrew McEwan (6/22/2009 3:07:00 PM GMT)
It is well documented that incumbent governments fair worse at local and european elections. They are often used for protest votes that wouldn't be cast during a Westminster election."