It would obviously make sense to have isolation in the flat, but common sense and electrical regs don't always go together as we know.
In your shower situation, hopefully the shower would have an isolator (as required by regs) - Hopefully that isn't in the cellar too!
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If the property is sold separately to the shop though in the future, the CU might become impossible to access so obviously that is an issue that really should be addressed, though which regulation it would fall under might be an interesting discussion.
Might be more sensible to get the landlord to contact the DNO and see if the service head can be moved into the flat? Though I doubt they'd do that FOC.