Step A. Become Prime Minister of Britain, Step B. More funding into Space Programs, Step C. Offer lucrative mining contracts in the Asteroid Belt to private companies using our well funded Space Program, Step D. Wait for a lack of resources on the Earth, Step E. Construct first Space Warship Fleet armed with Nuclear Ordanence, Step F. Using resources from the Asteroid Belt construct a space station near the Asteroid Belt with a small detachment of the fleet to destroy any asteroids that head for the station, Step G. Move majority of populace in Britain to new Space Station, Step H. Nuke China and America and force the submission of remaining nations, Step I. Give equal rights to all populace on Earth, policing being conducted by orbital bombardments on areas of resistance and terrorism. If it gets out of hand the Space Station already has a large enough viable population to rebuild the human race without genetic failure thus wipe the Earth clean and have it only as a religious base. Step J. move out into the wider galaxy and find Goldilocks planets to colonise, Step K. Create the new Human Empire.
Completely mad idea i know and relying heavily on space travel etc but slightly viable, just need to find out how to recycle atmosphere, CO2 to Oxygen but apart from that even just using a fission drive on the Warships will provide enough power to do a lot. The later steps rely on the fact that we have researched enough methods to have viable FTL methods.