In message
,
Chris writes

Why can't they charge off-peak cap on travel prior to 0630, if no
further travel is done until after 0930 (and before or after the pm
peak)?

They do. See: http://www.tfl.gov.uk/tickets/oysteronline/9261.aspx

"If the total cost of your journeys is less than the peak cap, you will
be charged separately for any journeys taken during peak hours, plus the
off-peak cap."

--
Paul Terry