Some reasons why Britain is still a world power:
1. It has nuclear weapons, and the means to deliver them anywhere on the planet;
That alone makes them a world power.
The U.S. the only true world power but the UK is in the second tier which is nothing to be ashamed of for a tiny island. I would put them there with China, Japan, Russia and Germany, and the UK has the highest quality military of that bunch.
Along with Pakistan and India.