If the cost of a one way ticket back is reasonable (and sometimes it is) just use Avios for the outbound. This type of situation does make me thankful that as a gold card holder I at least have the choice of using double Avios to force availability on reward tickets. It's particularly useful for European destinations during the summer.
In any case, this is often common on US carriers, too. Maybe all the southern Europeans are connecting via London on a trip to the US or Asia this summer.