Cancelled passport at UKVCAS appointment

We have a UKVCAS appointment booked and coming soon.

I understand we need to take identity documents with us to the appointment.

I applied for my 9yo son's Australian passport to be renewed months ago but they have major delays in processing times. I am worried his new passport will not arrive before our appointment.

When I applied for the new passport the Australian embassy cut his current passport through the machine readable code on the photo page, so it is no longer usable.

He does have a BRP card issued by the HO, showing his ILR.

Will the UKVCAS team accept his BRP as proof of identity or will they want to see the cancelled passport too?
I am wondering if they will consider this a valid form of ID.

UKVI does accept applications with expired passports (or even no passports) for both children and adults, UKVCAS process the biometrics and documents.
I would take both BRP and old passport.

I think you will be fine with the BRP and expired passport. When I had my my UKVCAS recently I observed my international passport was not even checked at all . I was only asked to show my BRP