Not every TOC is a Penalty Fare area (also known as a compulsory ticket area), where you must buy the ticket before boarding.
Not entirely sure, but it's governed by legislation so I think it's a combination of the Rail Delivery Group (TOCs) and DfT.
GB Railways is not yet in operation. We don't yet know how GB Rail will change the Penalty Fare system for sure (if at all).
The 2018 legislation applies in England and Wales; the 2022 amendment applies (the £100 Penalty etc) applies in England only as it is a devolved matter whether Wales wants to allow for this. As it is the Welsh Government chooses not to apply Penalty Fares in Wales, although they do in England (!). In the current environment it is down to the relevant contracting authority whether to make use of the legislation - there are no longer designated Penalty Fare TOCs within the legislation, it is a management/contracting matter.
GBR has no legal authority over TOCs without legislation (which isn't happening anytime soon); RDG plays no role in deciding this policy and there is no ATOC scheme structure for revenue protection. That leave the DfT to come up with an deploy a rational policy, which is why there isn't one (DfT being incapable/ill-equipped for this level of strategic management). Therefore as it stands, if you were doing Penalty Fares before your doing them now, and if not, you're still not.
The logical way to apply it would be on a journey basis (so, for example, if Penalty Fares apply in the West Midlands, it applies whether you use LNWR, WMR, TfW, XC or Avanti). But any kind of joined up thought is going to require a rail authority capable of making it happen, and that awaits the necessary legislation.