In case anyone reads this in future....

I was worrying about nothing. I was able to phone and cancel the booking without any difficulty. I have since found a reference to this scenario in the Guide to spending avios where it says the BAEC member retains control of the booking.

Avios and GUFs back in my account. Only downside, because I ended up having to cancel rather than rebook, was that I was charged the cancellation fee as I was not on the booking.