The OP title appears to be a little misleading. It should not read "second most powerful country in the world." It should read "second most in ability to project power globally". Russia, China, India, and lots of other big regional powers have a more powerful military, but they do not have the ability to project that power globally, outside their own region.
And nations project power with more than just their military. Nations also project power culturally, economically, politically, etc. Combine such "soft" power that has a global reach with "hard" power that has a global reach, and it is easy to see how the UK is number 2 overall.