From what I remember the sources of English law are delegated legislation, statute, precedent, custom and common law. Are you just refering to the practicalities of law making in the Westminster parliament because as far as any law making is concerned, nothing has changed for eons.
you need to decide: advantage or disadvantage to whom? A dictator would have much less trouble getting legislation through without having an opposition to deal with; the Westminster system is slower but does give people opposing view more of a say: voters can talk to their MP, who can raise the issue in Parliament.