Cutting off electrical supply to replace consumer unit
That is the supply fuse you cannot touch that. It is under the control of the Distribution Network Operator (DNO). It is a legal offence to touch it and it will have steel twist tags on it that have a stamped number on it which is recorded with the DNO so they know if it has been tampered. However after it should be the supply meters and then it should go to an isolator. You are supposed to turn off the isolators and and you can modify the installation after that point. However changing a CU is notifiable work in the U.K it is not something you are supposed to do yourself without being a competent Electrician.