Realistically you are going to have to install a cu to the 17th Edition. If the bathroom lighting is on the upstairs lighting circuit this will be protected by the incoming RCD, as will the rest of the circuit. You will no doubt have to upgrade the main earth bonding, but that's a natural.
Test and inspection will be on a new installation certificate but specify exactly what work has been done.
As I have said on a couple of other threads, before you have change the board, check if your landing light is off the downstairs lighting circuit and if it takes the neutral from the upstairs circuit. If it is disconnect and make safe the existing neutral and run up a neutral feed from the downstairs lighting circuit otherwise the RCD will keep tripping out. If it is an old property with just one lighting circuit for upstairs and downstairs you will not face this problem.