The barriers at Clapham Junction swallowed the ticket. It didn't have
an underground roundal; it was, visually at least, exactly the same as
any London Terminals to Clapham Junction ticket I've had.

The machine screen displayed "Standard day single - Paddington to
Clapham Junction", so if challenged that, coupled with the fact it let
me through the barriers and that the definition "london terminals" is
a bit weak, would be my defence.

I know when buying a ticket in reverse that it doesn't let me through
the tube barriers...