In other words, we are about to seriously stop pulling our weight, because we won't go to war without America?
More ominously (and interestingly) for Britain and America, the creation of a European superstate around a Franco-German axis where expanded majority voting makes it impossible to veto major initiatives means that Britain will find it impossible in future crises to go against the common "European" position as was the case with Iraq.
Under a future United States of Europe, the British government would nominally still control its foreign policy. But its ability to go it alone against the weight of the other USE member states could be severely compromised. If the EU foreign minister declares European opposition to a future war by America, the political costs of siding with the United States could be huge for a British prime minister. Even during the buildup to the Iraq war, Blair came close to losing his job by balking at the European consensus. Such foreign policy intransigence could possibly lead to marginalization within the European Union, with all the costs that could entail.
So, the question isn't will Britain go to war without America. The question is will it go to war without France and Germany.
Interesting times, indeed.