Both of those are very good questions-- and not nearly as easy to find answers to as they should be!
The list of areas where it is currently available seems to be here:
https://www.england.nhs.uk/diabetes/treatment-care/low-calorie-diets/ .
I haven't yet found any specific information as to when it will be rolled out to more areas. In fact, as I kept searching and not finding, I began to wonder whether they had any intention of further roll-out or whether they had decided to hit the pause button, which would be a terrible shame! ... And then, happily, I found these:
https://www.sell2wales.gov.wales/search/show/search_view.aspx?ID=SEP412101
https://www.contractsfinder.service.gov.uk/Notice/f44184ad-592c-4da3-8010-e2f229efee89
In September 2022, NHS England published a 'Prior Information Notice' saying they would soon be inviting tenders for contracts for providers to provide the NHS Low Calorie Diet Programme (the official name for the 'soup and shake diet') across the whole of England "to achieve full national coverage in 2023/24".
And then in November they published the invitation to tender, closing 22 December 2022, for "a Framework Agreement with up to 5 providers" with a view to contracts starting in February 2023 and ending in January 2025.
So-- it seems they do genuinely intend to expand the programme to the whole of England, and sometime this year or next.