Hi
If it was not activated, then its no good any more.
https://www.bt.com/help/account-and-billing/information-about-the-bt-reward-card
"The important bit - use it or lose it
Once we issue your card you have 12 months to use it. If you don't use it within this time, we'll remove any money that's left on it.
You have three months to activate your card from the time your product or service goes live. If you don't, you'll lose any money that's on the card."
Really gutted it landed in junk mail! I was hoping that BT might be able to help but seems unlikely 😞