The existence of a legal right does not create a body of knowledge or a reservoir of skills.
This is the official building regs guidance:-
https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/441669/BR_PDF_AD_B2_2013.pdfThe relevant pages are 93-95
As you can see, the cladding was required to be Class 0 (or its EU equivalent).
Here is the industry approval board certificate for the relevant cladding. The version used at Grenfell was the non-fire retardant (that has a technical meaning) version.
http://www.bbacerts.co.uk/CertificateFiles/45/4510PS1i1.pdfThere are one or two caveats.
Only one colour cladding was tested.
The tests were not of the wall plus cladding. However, how realistic is it when buying a building product to build a facsimile of a 40 year old wall, to attach cladding to it in order to create a fire test of wall plus cladding?
Subject to those caveats, the non-fire retardant product was certified to be (regardless of whether it actually was) Class 0.
If you had been the specifier would you have chosen this cladding?
If you had sat on the board of the landlord would you have accepted the specifier's advice to choose this cladding?
If you say, "no, I would have chosen the fire retardent cladding"; you need to answer the question "why"?
Do you only fly on Boeing 737s rather than 747s because 737s are safer or do you consider that both types of plane achieve acceptable safety standards? That certificate says both types of cladding achieve acceptable safety standards.