I think that journey falls within the old Network SE area so senior railcards are not valid before 0930
The network area restriction for the Senior Card on weekdays is 'not valid until off-peak day fares become available', which is
sometimes 0930, but not the case as you get further away from London. I use mine from Winchester on the 0856, for the first train to arrive in Waterloo after 1000.
Of course many people think that 'off-peak' is after 0930, but it is different for off-peak day return tickets and normal off-peak returns
Paul