I know this is complicated for Cambridge to London, with the 09:20 available
for off-peak day returns but off-peak returns not valid till 09:30, meaning
the 09:50 is the first available train these days since the 09:30 is now the
09:27.

Looking up a day return fare across London via Victoria I noticed I was
offered journeys via the 09:27 but not via the 09:20.

Is the rule different for trips across London via SN? I was looking at
Cambridge-East Grinstead if it matters.

Other SN destinations (East Croydon, Crawley, Littlehampton) seem to offer
journeys via both the 09:20 and 09:27 so maybe it is just connection
allowances across London that cause it to ignore the 09:20?

--
Colin Rosenstiel