If you say “The Grenfell Tower holocaust”, nobody - NOBODY - would genuinely believe you mean ‘the mass murder of Jews at Grenfell Tower”, everyone would understand that you were referring to the devastating fire there which killed 72 residents.

‘The Holocaust’ = the murder of six million Jews and other minorities by the German Nazis in the 1930s and ‘40s.

‘a holocaust’ = destruction or slaughter on a mass scale, especially caused by fire or nuclear war.

It seems perfectly simple to me and, I’d venture to suggest, most other rational people. End of, AFAIC.