Final answer:
John Locke is the British philosopher who advocated the ideal of constitutionalism, stressing the importance of governments existing with the consent of the governed and the protection of natural rights.
Step-by-step explanation:
The British philosopher who advocated the ideal of constitutionalism is John Locke. Locke's significant contributions to the political philosophy include the concept that governments exist with the consent of the governed and the importance of protecting the natural rights of individuals. John Locke is particularly well-known for his influence on the American founding documents and his arguments for natural rights, such as "Life, Liberty and Property". The ideas he put forward became founding principles of modern democratic states and greatly influenced representative government theory.