Before working on any electrical circuit isolate the supply by switching off the main switch and remove the fuse to the circuit that you are working on, If it is a circuit breaker switch off the main fuse and circuit breaker and remove the wires from the top of the circuit breaker. Place a sign saying that you are working on the circuits on the consumer unit, then nobody will restore the power whilst you are working on it.
If you are not 100% certain what you are doing call a qualified electrician. Building regulations are changing all the time and modifying your home electrics could be against new rules and could invalidate your home insurance, if in doubt check first!