The Framers of the American Constitution attempted to build "checks and balances" into the foundation document. There was a deliberate attempt to create a separation of powers - building upon the warnings of Montesquieu - echoed by Madison who wrote that "the accumulation of all powers, legislative, executive, and judiciary, in the same hands, whether of opne, a few, or many, and whether hereditary, self-appointed, or elective, may justly be pronounced the very definition of tyranny"
Other "checks and balances" include requiring special majorities or procedures to change the law - sometimes referred to as "entrenchment"; requiring confirmation by the legislative branch of executive branch nominees; control of the raising of taxes by the legislature...
The UK has its checks and balances. It has relied heavily on unwritten rules of behaviour which are enforced by social pressures. Conventions have been defined as "rules which are not legally enforceable, but which are regarded as binding by the constitutional actors". Some have been surprisingly effective. The House of Lords prides itself on "self regulation" - again which seems to work in practice.
But "checks and balances" can over time lose their effectiveness - the challenge for Britain and America - and every other country; city - even club - is to ensure that any system of checks of balances develops to continue the effective control necessary.