Are you being given an option to cancel and apply for a voucher, or to fully cancel the booking and refund? (I haven't seen functionality for the latter on their site since pre-COVID).

If the former, I seem to recall that points refunded immediately, and the cash took a few days. I can't answer the question about whether cancelling online would impact what you should get back - in theory it shouldn't, but BA IT... if you're not certain, call in.