dangerous light?

i am hoping to put in a shower but there are no spare mcbs ( i think thats what they are called) in the consumer unit to run the supply from. I do not want to go to the trouble of upgrading my consumer unit if i can help it. Someone suggested putting the upstair and downstairs light on one mcb and then takeing out the redundant mcb and replacing it with a higher voltage one for a shower. Is this safe to do and legal. I know that the whole house would be plunged into darkness if it tripped.

I would like to divide this job up into two different phases and was looking for advice on how to do this. I currently have a 100-amp panel located in the garage in a home built in 1972. The meter is over 100 feet away and I have decided to upgrade the panel and meter socket to 200 amps. I have called the Power Company and they will replace the 100 feet of wire coming into the house (for a fee) and I am to replace the meter socket with a ringless and open up a 2-foot deep trench for them. They will move the meter the 100 feet to the house t the new meter socket. The new location of the meter socket will be about 3 feet from existing panel. Phase 1 Have the Power Company come out and pull the meter. I will then pull service feed out of the old panel. Install a new 200-amp meter socket and put the old service entrance cable on one side of the meter socket. The other side of the new meter socket will feed the new panel. I will also install the correct size of ground wire to the water pipe for 200 amp and install a new ground rod outside for the panel. Have the power company put the meter back in the old meter socket so I have power. Have it inspected. Phase 2 Run a 2 foot deep trench for the power company and have them come out and replace the line to the meter socket and move the meter to the new location. Questions: There is a 1-1/2 pipe running from the outside wall to the old panel can i reuse it for the copper wire to feed the 200 amp panel? Is there a way to jumper the new meter socket temporarily? Is there a problem with the service entrance wire from the new meter socket to the old meter socket being protected by a 200-amp breaker temporarily? What are some other things or ideas I left out? I am going to take the plan to the inspector before I pull the permit but wanted to make sure it made sense. Thanks