You have to got the BT shop,
Register there and they will give a code, it's not the main BT site.
Also, expect to wait... it took them 3 days to do a "security check" and a further 10 days after the check to process the order. Basically. taking it off a shelf and putting it on a van.
Check your email people, as they don't tell you that before you order.
They have come in and out of stock at Game in that time
It was easy ordering from BT I have to admit, I kept on getting told it log in. I rang BT however they were saying it was my devices and had to change passwords. It was only when I logged into my account details that I seen the activation code for a PS5 order. They had sold out for the basic console model, ending up buying one with 2 controllers. Still got it cheaper than other sites I was looking.