I had thought that the return could be cancelled prior to the outbound flight being taken and this is confirmed by a BA employee in this thread (although others disagree that this is possible) and again in this one.

The outbound is early January and I suspect there's zero chance of it going back into award inventory. I don't think I can risk that.

However, on reflection, I may be able to push back the return and use it later; what do we think my chances are of getting through to an overseas call centre, explaining what I want to do and nabbing the new flight (for next December) on a very popular route on the stroke of midnight? Not good, I expect.

And for info, Boraxo, the Amex 2-4-1 can be used one way from the UK, at least in practice (I've not checked the rules). In fact, the usual way to get the in demand routes is to book one way online at T-355 and then add the return when it comes into range.