The Tory symbol was a stylised torch. However in the past few weeks Cameron has replaced it with a scribble of an oak tree. You can see the image of the new logo in the Wikipedia article

http://en.wikipedia.org/wiki/Conservative_Party_%28UK%29

The Liberal Democrat symbol is called the bird of liberty and was once derided by Mrs Thatcher as a dead parrot. Obviously her speechwriter was a Monty Python fan.

The Labour Party used to have a red flag logo, but that was thought to be a bit left wing, so the red rose replaced it.

I suppose the parties provide some work for graphic designers.