My understanding is that it is not a legal requirement to take your PNB when scheduled.
Correct. The legal requirement is that you have to have a break and you cannot work for more than x hrs. Ergo my comment on flexibility earlier. Sheduled breaks can easily be missed if you meet the legal requirement and can have a break later. That may also affect another service because diagrams are too tight. Sacrifice one service for another ? That is certainly NOT your decision and is down to control. You may want to forgo our break and are willing to take it later but control state otherwise. It's a catch 22 situation.
However the consequences of not doing so *may* land you in legal hot water if you are subsequently involved in a serious safety of the line incident and *will* result in you being allocated the lion's share of the blame for any such incident.
Its odd because whilst you do meet the legal requirement it still becomes an issue and will 100% be mentioned during an incident. Breaks are often placed on a diagram to meet fatigue requirements and skipping expected meal breaks can lead to distraction (thinking about your break rather than driving etc.) Again your in a catch 22 because you know in your head that you may not get it later, or any incident it becomes a salient point of investigation. I always say to colleagues that you absolute must ensure you are well rested and your break that gets deferred is 100% taken later in the diagram. Remember you may be involved in a delay and your break pushed further back. Again, it is a factor to consider.
Skipping/delaying your PNB for additional pay is therefore a personal decision which must be carefully made, with reference too your own assessment of whether your performance is likely to be significantly impaired by not taking the break. It is essentially a decision that comes down to professional judgement.
Only at certain times. As pointed out by others already. When you reach that critical point where you are REQUIRED to take a break then you cannot give that up for remittance of any sort. At that point it is a legal and safety critical responsibility. Professional judgement is to not take the service. Work safe ALWAYS