Companion vouchers require all flights to be on BA aircraft. From what you have described your connecting flight is likely operated by another airline (even if it has a BA flight number).The only way out of that is not to use the companion voucher, or to get the connecting flight as a separate ticket. However, if you get separate tickets make sure you understand the many problems of doing so, like having to collect and recheck luggage between flights, and no protection for delays on separate tickets.