To those lucky enough to have attended school when a subject called 'history' was actually taught, the word 'Quisling' had really only one definition and that referred to a Norwegian politician of that name who helped Nazi Germany conquer his country and subsequently governed.
The Concise Oxford Dictionary provides a definition of 'quisling': a person cooperating with an occupying enemy, a collaborator or fifth-columnist, a traitor. From Wikipedia we learn that "Quisling" is synonymous with "traitor", and particularly applied to politicians who appear to favour the interests of other nations or cultures over their own."
When considering the principles of British politicians today, coupled with the Parliamentary Oath taken on becoming an MP and the Privy Councillor Oath which those MPs who attain that exalted position take, and the position in which this country finds itself, viz-a-viz the European Union, perhaps the term 'quisling' can be applied to Cameron, Brown and Clegg. Whilst the EU may not, presently - but give it time - have a physical presence, it is most certainly 'occupying' Britain in that the country is subservient to so many of its laws.
For those MPs, who are not Party Leaders, the charge of 'quisling', it could be said, carries more importance as they have slavishly followed their Leader's, and Whips', diktats and voted as they were instructed. In so doing they have put their careers on a higher level of importance than their inherent duty to do their best for their constituents and, ultimately, their country.
Someone once said that politics is a dirty business - it seem to be also one without honour!