Before I attempt to answer the question, a couple of observations.
A Harvard professor coined the term "soft power" to describe the influence of an organisation or country beyond what might be expected given it's military or economic ability.
A recent survey (by the influential Monocle magazine) appears to show that Britain is the "most powerful" country in the world in those terms.
A "Rolls Royce" diplomatic service, influence within the EU, part of NATO, nuclear-armed, connections in the old commonwealth/Empire, cultural and sporting successes, the fastest growing economy in the Western World, effective centre of the international money markets. (And there's more than that of course, but you'll get bored pretty soon).
The second observation is to ask what is a superpower? Does that include China, the second-largest economy in the world, a country on which we depend for most of our manufactured goods, but which has an poor military, who's citizens are for the most part dirt poor and which has a huge and unsustainable debt. They have started building aircraft supercarriers.
The real answer to your question really depends on whether Britain actually wants to be a world power again. There is no doubt that with a growing economy, and a diversion of funds, Britain could once again "grow" it's military power.
To get back to the situation at the end of the 39-45 war, where we had a huge navy, huge airforce and a milllion men under arms, would take 20 years. Would the rest of the world allow us to do it?
What would we use such military power for? To reinvade all those countries we've given independence to, to recreate the "Empire"? To become the "worlds policeman" again?
History tells us that all major powers, without exception, fail eventually.
We had 150 years on the top of the pile but I suspect our time has gone forever. Today belongs to the USA, Tomorrow belongs to China.
After that? Global Islam?
