On 5 Dec, 09:27, wrote:
if it's raining I might want to get the Tube to
Shepherds Bush and then WLL - you get charged twice, more than if you
took a journey of equivalent length on the Tube, even changing lines.

No you don't. Oyster PAYG fares are based on the start and end points
of your journey. If you change trains at a recognised interchange,
even if you have to go through barriers or out onto the street, it
counts as a continuation of the same journey. In other words, in your
scenario, the second leg is free.

U