@plugwash has given a very good reply, when mothers kitchen was altered when she became disabled there was a problem with the house electrics, the house needed rewiring, but dad put his foot down and said I am not living in a building site, you can rewire it when I'm gone, so needed to find a way around the problem.
The method used was a steel wire armoured cable fed from a large MCB in the original consumer unit and a second consumer unit in the kitchen which just powered the kitchen.
There is always a way to do it, but not always what we really want, in mothers case there was no option but to fit a new kitchen as work services too high for wheel chair use, but had she not lost her leg, then the rewire would have come first.
The problem is until some one has used meters and tested the installation we have no idea if you can simply fit RCD's or RCBO's. We have two main meters, the clamp-on ammeter with a 0.001 amp scale so we can test actual leakage, and the insulation tester which uses 500 volt DC to calculate the leakage, until tested you may have unknown faults, this house in the hall some one had selected the wrong neutral in a 4 gang switch, and upper and lower floor lighting would not work until corrected, lucky just one wire to move.
Many houses need the stairs lighting rewired or wireless switches fitted where lines were borrowed when first installed, referred to as borrowed neutral.
When mothers wet room was being installed (around 2004) I got a phone call from the electrician, he was to fit a second consumer unit with a RCD just for wet room, and for an extra £100 he would fit a new consumer unit for whole house and then whole house would be RCD protected. (This was before the kitchen)
I got to mothers house to find the electrician pulling out his hair, and could not get the RCD to hold in, he as a temporary measure fitted an isolator instead promising to return next day, never saw him again. At least back then not forced with regulations to fit RCD's so technically broke no rules.
Daughter also had problems when fitting RCD protection, it took me all day to find the screw in a socket which had earthed the neutral, so easy to fix, but so hard to find.
OK daughters the electrician had identified the fault before starting, and made an agreement with me I would come next day and fault find. But an electrician should do a part EICR before starting so he knows what the problems are.