I have just bored myself to death on the ONS site. Send mourneros.
For what it's worth, here are the links to where the ONS explains:
a) the plans to change methodology for CPI - November 2021. (ie well before Jack's viral thread)
Plans to incorporate new data sources and methods into the structure of UK consumer price indices from 2023, including changes to the existing hierarchy and methods of weighting different strata.
www.ons.gov.uk
But more importantly
b) the use of experimental data to track the cost of a set of budget items, a process which
commenced in APRIL 2021.
Results of which led to that Telegraph (?) article she was incandescent about in May 2022.
This experimental analysis looks at how the prices of the lowest cost products for 30 everyday items have changed since April 2021
www.ons.gov.uk
edit: I remember both these points have been made previously, but given the ONS response to the FOI request is doing the rounds, thought it helpful to link to supporting evidence that Jack does FA.