The current T&Cs are on the BA website. These are dated Feb 2022
Rule 11 states:
Cabin Upgrade Vouchers can only be used for British Airways flight-only itineraries with departures from the United Kingdom, and are available for British Airways mainline and British Airways CityFlyer franchise operated flights issued with a British Airways flight number only. Partner and code-share flights are excluded and will not be eligible for the Cabin Upgrade Voucher, including SUN-AIR.