How long are you planning to spend in the UK? Whatever it is, assuming some redemption seats are loaded onto the system 355 days before travel, if you want to book both legs at the same time you will have to wait for the return redemption seats to be loaded X days after the outbound before you can book both legs, at which point the outbound seats may (probably will) have already gone.

A better option may be to grab the outbound seats as soon as they become available and do the the same for the returns (single Avios tickets are exactly half of a return, unlike cash bookings). Of course you will have to be flexible with the duration and departure dates and may have to change the outward flights if no returns open up when you need them.

As others have said, the chances of four CW seats on the same flight are slim so you may have to be flexible; are you prepared to travel on different days or in different classes?

Oh, and welcome to FT!