My attention has been drawn to the social distancing one-way system at Finsbury Park which has broken the out-of-station interchange in some cases when changing between National Rail and the Underground. Access to the National Rail platforms is currently only from the bus station entrance. Exit from the Underground is also to the bus station, but entry is via the new entrance on the other side of the station, a 3-4 minute walk.
The problem is when passengers arrive on platforms 1-2 from North London and Hertfordshire. There are no gates on those platforms so you have to touch out using the validators at the bottom of the steps. When you touch in again at the new Underground entrance your journey is not continued because the OSI has not been set up properly. This means that your journey is split in two and you are charged two fares rather than the normal one combined fare. TfL are aware of the issue and I hope to have an update within the next few days.
In the meantime I suggest that to join the Victoria line you should use Highbury & Islington instead where the interchange is cross-platform. For the Piccadilly line beyond Kings Cross I’d take the Victoria line from Highbury & Islington and change at Kings Cross, or if your National Rail train is going to Kings Cross or St Pancras then stay on it until there.
There are no problems changing between the Underground and National Rail because the OSI works between the two bus station entrances.