I agree batts,
You change the CU you make the installation safer, you explain all the options to customer to sort out the problem, if they don't play ball issue danger notice and be done with it. If you don't someone else will do the same.
It seems a daft way of working, for example I come along to mrs jones, she asks for me to install a socket, I then explain about rcd protection, she decides on a new board, then her lights aren't earthed to needs lighting re-wired or changing. You can see how customers think you are having them on when you first only went to install a socket.
She thinks of all this extra cost and then decides against the socket sticking with her extension lead upon extension lead.
Now then whats safer, the extension lead upon extension lead, with no cpc in lighting and no rcd protection, or new socket, new board with rcd protection on everything, and at worst no cpc in lighting circuit with danger notice issued.