Welcome to FT Jimmy_uk and great to see your first post.

There are two ways to do this, either you book your outbound online or on the phone, then call at 1am to add in the return flights when they are released at 355 days. If you do it this way you would pay avios and taxes/fees/charges for the one way outbound first, and when you can to add in the return the taxes/fees/charges would be recalculated and you would pay the difference for a return, so up to £1,231.98 figure.

The alternative is to book the return sectors as a one way on ba.com at 1am, then call up later where the agent will apply across the companion voucher to the second booking and refund half the avios. The disadvantages doing it this way are that you need the avios to cover the return booked at full avios price before a refund later. Also the taxes/fees/charges are not recalculated so you end up paying taxes/charges/fees for booking two one ways, i.e. the £841 + £1054 you noted above. As taxes/fees/charges are quite high for exUS booking this is not a good way to do UK-US trips, however there are some countries where taxes/fees/charges are lower and it may be worthwhile doing such as Hong Kong or Brazil.

Here is my own worked example which basically says the same as you found https://www.flyertalk.com/forum/30255777-post965.html

If you haven't found it already, this thread is a very useful resource for information on using the companion voucher https://www.flyertalk.com/forum/brit...er-thread.html