i have only ever paid on collection day.
Been the same for me with every car I’ve had on the scheme, which has been with Vauxhall. Told the same for the one I have on order now with VW, too.
This is the second time I have used the same dealer. The first time cash was no problem. This time the same person said payment need to be by bank transfer. I requested that I wanted to pay cash again and in all fairness he went to ask the manager, who just told me they can’t do it anymore sorry!