That's a truly shocking install. Shouldn't this be in the black museum forum?Re the inaccessible CU, there really is only one solution, remove the kitchen wall units and do a proper CU replacement, updating bonding etc.
If the owners, or their builders, then choose to put the wall unit back (they probably will) that's their problem. Put the new CU as high as possible so they will at least be able to get at it to reset any trips, even if it will still be impossible to actually work on if the units go back.
Most kitchen wall units just hang on brackets or a rail at the top, so unless they have gone mad with mastic, it should be easy to remove them without damage so they can go back again.
I would definitely call the DNO and report what obviously looks like an unsafe supply and see what they propose. I guess technically you need not be concerned about the supply arrangements to other properties or the rest of the street, just the supply to the property you are working on.
Are you sure ALL that external wiring is on the DNO side? some of it looks too small to me and might be someone's domestic wiring on the outside of the building?
Regarding why this was not picked up on the sale. I would be advising the house owner to write to the solicitor that handled the purchase for them. Usually solicitors make it their business to check all aspects of the property and often spend ages investigating what look like trivial matters, so the question has to be asked why was this not checked out? why were certificates not requested, if they were not available did they bring this to the attention of the purchaser etc?
If it can be proven the solicitor was negligent, don't they have insurance to cover this? could a claim be made to cover the costs of rectifying the faults that should have been found by the solicitor?