Just noticed that BA have recently changed policy and will offer cash refunds for some vouchers not used within 2 years of issue. This is from their website.
Who is eligible for a refund?
If you requested an eVoucher after your flight was cancelled between 9 March 2020 and 19 November 2020, you can request a refund.
How do you request a refund?
Refunds can be requested via the online refund form. You can apply for a refund 24 months from the date of issue of your eVoucher. An email will be sent from us informing you when this is.
Can you request your eVoucher to be refunded if it’s not yet two years from when it was issued?
Refunds can only be applied for 24 months from the date of issue of your eVoucher. An email will be sent to you to inform you of when this is. You can use your current eVouchers to pay or part-pay for flights that are completed by 30 September 2023.