Hi all

I am looking at booking LHR to SIN for my parents, with a companion voucher, for the 20th Nov 2025.
I will also then book BKK to LGW for their return leg, and call to have the avios etc refunded so I can link the bookings.

What's the best / quickest way? Will it be to call BA at midnight if there is availability? The lines close at 8pm, should I call their US office perhaps?
Thanks