The primary reason for the existence of a state is the defence of its territory. This is true for all political primates - be they chimpanzees, baboons or humans. To defend this island nation we need, and have always needed, a navy. Since the advent of the aeroplane we have also needed an air force.
As to adventuring overseas, the question is simply what, if any, political purpose is being served. Apart from defending ourselves from attack by another nation or deterring such attack in the first instance, the only logical reason for having the capability to use force of arms beyond our own borders would be the protection of national interest - seizing the territory of others so we can exploit its natural resources or preventing other nations from interfering in our economic and trade affairs. The size and composition of the military depends upon the use for which it is intended. The current use in Afghanistan is at best questionable and the invasion of Iraq was wholly unjustified.