.. .
The gates at London Bridge on the National Rail platforms will let you
through with only prepay on an oystercard, but will not deduct any
fare or resolve the journey. The validators on platforms 5/6 must be
used even if you have already touched in or out there.

That could be useful to know, if only to avoid putting my season ticket
through the gates.

Frequent use may well be monitored to stop "dumb-belling".

Does anyone know the position with the gates/validators at Farringdon? A
typical scenario is an occasional journey Brighton to Moorgate and return:

1. I use my National Rail season which is valid as far as City Thameslink.

You are travelling without a valid ticket between City Thameslink and
Farringdon. To combine a paper National Rail ticket and Oyster Prepay
you must touch in where your paper ticket ends, at City Thameslink.

2. At Farringdon I validate the prepay card on the platform (does it matter
which platform I do this on?) and then
3. Change to the Underground for Moorgate.
4. At Moorgate I the prepay card lets me out through the gates.

As far as I can see this all works as I would have expected, in both
directions. HOWEVER, I once found that the delays on the underground at
Farringdon were going to be excessive, & thus decided to leave the station
and walk - but after I had validated the prepay card. The oystercard refused
to let me through the ticket gates ...

Prepay requires a touch in and out, so your usual journey is valid
between Farringdon & Moorgate. Just touching out at Farringdon would
lead to an unresolved journey.