From the outside lookin in, easy for me to say because I got no skin in the game, how I would have handle this i dont know
but what i do see neither responded with 'grace' and it escalated each time, now its ugly and nobody is a winner.
they do have a refund policy, I think a simple request to return for refund would have been an easy result, their website indicates they are a dropshipper, so perhaps thats what happens for out of stock items.
I dont think you have a case to take up to consumer affairs, because you havent actually asked to send it back for a refund. Perhaps the best outcome would be to start again, write back and apologise for overreacting (say the cat died or something) and ask for a return for refund. Sure you are sucking it up, but whats the shame in that, nobody will know and you get a win, I cant see a win any other way.
https://gammongolf.online/returns-policy/
GAMMON GOLF RETURN/REFUND POLICY
All returns must be within 14 days of receiving item
Sale items may be returned for store credit
Store credit cannot be used in conjunction with any other promotions
Full priced items may be returned for a refund
Any faulty or incorrectly sent items will be fully refunded