The UK has a well-established constitution. It is forged into the national culture and baked into the brains of the people, and, from time to time, based on bits that are written down. The government has checks built in. The US concept is like a tug of war, with branches opposing each other. The Westminster concept does not set power in opposition to itself, but binds those forces together in tension, more like contestants in a three-legged race.