Some nice ones in here. My favourite might be "absolute parcel." I've heard Brits and Aussies both call someone a muppet, but I'm not sure how to use it myself, even though people use that one stateside as they do "donut."
I'm posting this here because I first encountered "pillock" here. It was none other than Sean Mac making the required dig at Gary Numan. I also like "unit" and "absolute unit."