That assumes members of the legislature always vote with their own party, which they don't. Plus, there are usually two houses and the upper house can scrutiniz

That assumes members of the legislature always vote with their own party, which they don't. Plus, there are usually two houses and the upper house can scrutinize and hold to account the lower house. In the UK it's an appointed (and partially hereditary) upper house, so there's less party discipline (although not none).