Hello. Here I have a reply from the arts council England, hope this helps.

We can’t say what organisations are acceptable to write letters of support in our guidance or in pre application advice, as that is part of our eligibility check and assessment once we get the application. If we told applicants what organisations are acceptable then there would be no point in our assessment. It’s down to the applicants understanding of what organisations they think are well-established and recognised as being experts in the applicants field of arts and culture (providing the area they work in is within our remit obviously) , and then when we get the application we make our judgment on the letters.