I really enjoyed this episode. I watched this years ago and have a theory about the plot of this film that I feel makes some sense and might solve a few of the massive plot holes. I do a very boring job and have plenty of time to waste at work thinking about this stuff. I apologize in advance for the length of this post.
Ok my first point is about the engine the ship used. The NERVA (Nuclear Engine for Rocket Vehicle Application) engine is an old cold war project from the 1960s which used a nuclear reactor to heat fuel and expel it from the back of the rocket at high speed to give thrust. While the engines weren't designed to give months of thrust as stated in the film with a nuclear powered reactor and a low thrust you could keep accelerating for a lot longer than traditional rockets. Also if you do have the ability to thrust for six months continuously the shortest journey time is almost always achieved with a constant thrust for half the distance and then flip the ship and thrust in the opposite direction for the other half of the journey (deceleration is just a negative acceleration in physics they are both classed as an acceleration). The propellant is the real problem since the ship is too small unless they dropped the propellant tanks during the journey as they ran empty.
But back to the main plot. I see it this way. The vampires arrive in the solar system at some point in the distant past low on energy and needing a top up. They can't just transfer energy from any animal or they would be able to use energy from a herd animal as it would be simpler and they only seem to use the energy from humans in the film you don't see the energy being stolen from infected dogs for example. So they alter the genes of early humans to make them compatible and wait for the human race to grow and hide in the comet as it has a relatively stable orbit. Then every few hundred or thousand years they come back to earth and harvest more energy to keep the ship going. The problem is that there method is floored.
Basically the disease which converts the humans so that they can collect the energy kills the humans if they don't feed within 2 hours so the contagion would not spread far in a pre-industrial world and their time in orbit would be limited by the need to get back to the comet. So they hibernate until the population has grown enough to allow lots of people to be infected in a short space of time giving them a lot of energy so that they can leave.
This brings us to the time the present day. Most of the vampires have given their energy to keep the three to vampires alive. The three vampires produce a glamour which will entice the crew to take them back to earth and then to leader mixes some of her life-force with the commanders to make them compatible with the human life-force. Draining the rest of the crew to give them strength to act. Once on earth the head vampire starts the infection in London and then acts as a distraction to draw investigators away. The other two wake up and instantly leave their bodies to hide in the guards so that they can act unsuspected. One tries to kill the doctor who researches life energy and is the biggest threat but is killed in the attempt. The other goes out into London to prep for the collection of the energy.
At this point London goes to hell as the infections spreads. The collection works in three stages. The first is to use the infection to convert the humans so that they can release the energy (turn humans into batteries) not into the vampire species. Then since the beam from the ship can only collect the energy from those near the beam the male vampire acts as a capacitor flying around the city collecting the energy from the rest of the infected in London and brining it back to the beam to be transmitted. Finally the head vampire needs the human commander from the space ship to convert that energy so it is compatible with the ship and triggers the transmission back to the ship.
I hope this helps making sense of the movie.
