It's a bit of a grey zone in my view.
Under the PRO:
The passenger shall not have any right to compensation if he is informed of a delay before he buys a ticket
There isn't an equivalent limitation in the NRCoT, however some operators do include them in their Passenger Charters. For example LNER's Charter says:
If there are delays and cancellations we will always try to tell you before you buy a ticket, so we don’t normally accept claims if you knew there was disruption and still decided to buy a ticket. However, we treat each claim on its merits and will never automatically rule out compensation
Given the lack of an NRCoT limitation you arguably have a right to at least the minimum entitlement under the NRCoT (50% of a single for a 60+ min delay) even if you knew about the delay - since the NRCoT entitlement is a minimum which operators cannot exclude in their Charters.
OTOH, TOCs like LNER which include a limitation in their Charter may argue that their limitation clause purely reflects the
de facto legal position, and that it's an implied term of the contract that you can't claim for delays you knew at the time of buying your ticket.
How this interacts with PAYG is very unclear. I've had compensation from GTR and TfL on a number of occasions when using their respective PAYG schemes, despite having touched in after the disruption was already advertised (indeed, after the advertised departure time in some cases). I think it's recognised that with PAYG you don't "buy" a ticket at a specific time but just touch in when you enter the barriers, so it's not really practical to impose such a limitation.