In article ,
Paul Corfield wrote:

Oyster cards have a very limited applicability to the issue under
discussion. There is a limited amount of Pre-Pay acceptance on the FCC
lines across zone 1 for Thameslink and Moorgate / KX - Finsbury Park for
the Cambridge / Peterborough lines. The only other ticket in Oyster
format would be a Travelcard based season which is not affected by the
new restrictions.

My reasoning is that when I go to London, I can buy a standard return (and
therefore use peak time trains), and use the Oyster for Underground travel.
At the moment I'm buying a day return from Cambridge to London/Travelcard
combined ticket.