1. Very much depends how the shower arm is fixed into the wall, and whether or not you are prepared to allow pipes on show.
2. If the shower arm is screwed into what is known as a wall plate elbow, then you could:
2a. Turn off all water.
2b. Unscrew the shower arm. Might require shower head removing first.
2c. Use a nipple, such as Screwfix 71087 to bring the thread out as a male 1/2" BSP thread.
2d. Use chrome plated 1/2" to 15 mm compression elbow, such as Plumbers Mate 25461502 to turn downwards.
2e. A piece of chrome plate 15 mm copper pipe to connect to a wall outlet.
2f. Wall outlet could be wall mounted shower connector like 15 mm x 1/2" male chrome shower elbow on Amazon.
2g. The chrome pipe would need to be fixed to the wall with chrome pipe clips. Depending on what size you get would alter the distance of the pipe out from the wall, and hence top and bottom connections.
2h. Everywhere with a screwed, rather than compression, fitting would need the threads sealing. PTFE, Loctite 55 or Flomasta Pipe Seal (or Loctite 577), the last being the most reliable but least forgiving.
3. If the shower arm is fixed in a different way, the top connection would need to be different, but no way of telling without knowing what you have got now.