Ahh good point, but then again there was no strong state structure until the Normans came. After the Normans established themselves, there were no more invasions of the British Isles by an outside power, primarily thanks to your fleet. I think this allowed you to develop under less vulnerability than say the French who bordered the Germans, the Balkans who got swallowed up by the Ottomans and Hapsburgs, the Germans who have invaded everyone and by everyone, and the Russians who experienced one of the most bloody invasions in history only 60 years ago. I do observe that sea empires are ruled differently than those that are land-based.
Your islands have not experienced invasion from an outside force for a long time, for almost 800 years and after unification with the Scots your biggest power on the island was absorbed into a unified state. I do think that THIS does contribute to how your country developed. If your country has been under threat of invasion or even occupation for most of its existence, the ruling powers are going to enact laws and regulations that could be considered extreme but vital to the continued survival of the state.
Look at Japan. Yes they had an Emperor, but usually it was the feudal lords who held the real power. This is another example of a maritime state having many different poles of power within their internal affairs. So I do think there is some legitimacy for the theory. FYI, I am not trying to discount the hard life lived by Britons, I am just trying to put Britain in comparison with the continental Europeans, who lets face it, have seen more war on their own lands than Britain has.