There are some differences between "the formation of cliques with a common agenda" and parties. Two important ones are that the parties have relatively fixed membership and are few in number, whereas cliques are both more numerous and more fluid - you still get cliques within parties at the moment, for instance, and for certain matters even across parties. However, the most important difference is that in our current system the party has a near-monopolistic control on whether an MP gets on committees, becomes a minister, is reselected for the next election; in short it has a huge say in the MPs future. Cliques can do that to some extent, of course, but as long as there are a goodly number they will compete and none will have the monopoly.