Apologies if I missed it, but do we know the official rebooking/rerouting 'rules' for cancellations that are kicked in to the long grass?

My LHR-CPT cancelled, I'm planning to rebook for 2022 but logistically it'd be much better if I could move to LGW instead of LHR (at least the return leg). Not sure if that's reasonable/within the rules to ask for or not since it'd add a non-BA leg (e.g. CPT-DOH-LGW on QR/BA rather than CPT-LHR direct on BA). Or alternatively get in to LHR later (but a day earlier) - although that adds the same issue with additional carrier I think.