1. Replace the switch fuse like for like. Leave the property no more of less compliant than it was before.
This would clearly be OK as it is the design date that matters, not the installation date.
2. Replace the switch fuse with a device incorprating a RCD. Satisfying the requirements for RCD protection of concealed cables, but leaving the whole installation on a single RCD.
This would be altering the design, so would need to comply with current edition, and one would need to measure leakage and do a risk assessment.
With my own house I know the combined differential for all circuits is around 20 mA I out of interest when I got my new clamp on, tested it around both tails to see the combined leakage, we are for a 30 mA RCD allowed 9 mA, so with my home this would not be an option.
However I have lived in a caravan with two 30 mA RCD's in series and had no real problem, what we must remember is AC can transfer with capacitive and inductive leaking, so using an insulation tester tells us very little as to if the 9 mA is likely to be exceeded, and until I got a clamp on to measure DC the first increment on my clamp on was 10 mA the next being 20 mA and I could have not been reasonable sure if an installation was within the 9 mA (one 1/3 of the RCD trip value) so could not know how likely nuisance trips are.
3. Replace the whole submain with SWA, massively expanding the scope of the work.
This is of course an option, be it alitube or SWA, and is what was done with my mothers house, the house at the time still had 1954 wiring, the council wanted to re-do the kitchen so she could use it in a wheel chair, so work tops were lowered and the kitchen needed wiring alterations, I had earlier tried to fit RCD's and they had to be removed as would trip on a regular basis, at that time they were not a requirement, so replacing with an isolator was OK, only the shower was RCD protected. But by time kitchen was done RCD was required, so the SWA ran outside the house from the original CU with a 45 amp MCB to a sub CU with all RCBO's.
The use of SWA resulted in no problem running the cable exterior to the house.
But if you are going to alter any design, then you need to do the risk assessment for the whole effected installation, I have seen where some one has altered a design without first testing and have split the lighting for example upper and lower floor as the ceiling rose only rated 5 or 6 amp, and they have fitted silly down lighters in the kitchen, so need more power. Fitting a 10 amp MCB would not comply as ceiling rose used as junction box, so they have found two cables in the CU so split them, on to 2 x 6 amp MCB's, however the result was a borrowed neutral with the stairs two way lighting, which was only found when it came to fitting RCD's. Even then seen it where so called electricians have simply put both MCB's for lights on the same RCD, this must be worst, as the first electrician who split them may have possibly not realised the two way switches borrowed a line in one two gang switch, but when it tripped the RCD the fault was clear, and so was cure, putting both circuits back on same MCB would have just returned to original design. Although using kinetic light switches would have been better.
But however it is done, when one alters a design, one must ensure it complies with regulations current at the time of the new design. And even splitting upper and lower floors lighting circuits is altering the design, as of course is swapping a single pendent lamp for multi down lighters.
We should of course raise minor works certificates to swap from single pendent to down lighters, what I am not sure about is if a compliance certificate is needed? Is work in the ceiling void of a kitchen considered as work in a special area? No problem in England kitchen no longer a special area, only bathrooms, but still is in Wales.
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