Firstshop doesn't want to refund for unavailable stock item

I ordered Navasolar on 6th May, and they sent me an email to say the item is out of stock on Monday, 11th May. Mind you, I called them first, and they assured me the item is in stock the same day. On Monday, I got an email saying the item is out of stock, and they requested my ID and proof of account (I generated a stamped bank statement), which I sent. I got an email requesting an account letter, and this time I downloaded a confirmation letter and sent it to them on Wednesday. When I inquired about the refund, they always told me they were waiting for feedback from their finance department. This is frustrating me as I needed to buy other batteries, which are now out of stock. If they don't have stock, they should refund people immediately because holding onto funds for unavailable stock violates consumer rights.

This is the last email I got from them @16:07 PM today. <<Hi David,

I hope this email finds you well,

We have requested feedback from our finance team regarding the refund and are currently still awaiting an update from them. I have followed up with them once again and will provide you with feedback as soon as it becomes available.

For your reference, your ticket number for this conversation is 14#####. >>

They promised to send me proof of payment on Monday, 11th May. This is so disappointing.