It depends on the situation. Don't forget that on most preserved lines there is a very different operating hierarchy to BR and as a result some job roles have become blurred.
On the GCR for example signalman technically do not have access to the locking rooms, but in practice a key is provided in the box because it's where the gas/electric/water isolation points are located, and we can't afford to lose a box for a matter of keeping with the old times - Network Rail generally seem to provide signallers with keys for locking rooms nowadays however.
However the old adage where an S&T technician cannot be a signalman remains.
There are alarms on the block instruments etc to stop anyone from tampering with them (very loud ones at that!), that require a key held by the signalling engineer to reset.
Competencies become blurred because people don't just do one job role - I'm qualified as a signalman, station master (chargeman/station supervisor/duty manager rather than the traditional olde style BR/Big 4/Previous station master role), shunter, S&T (installation and supervised maintenance only) and so on - as a result I hold more competencies than a traditional Signalman might have done, and in an emergency I can act appropriately (ie if I'm a duty signalman then I wouldn't get any further involved in the S&T side of things than going to see if my detection/FPL etc had got stuck on the points, as any good person should, whereas if I'm on duty as a booking clerk and something goes Pete Tong then I can weigh in as required after informing the relevant people).
It's also worth remembering that many senior line managers are volunteers themselves and not always on site - there is a nominated Responsible Officer who acts as the senior duty manager on the railway, but they are not supermen and can't be expected to be doing everything - why is why the GCR rules allow for them to nominate competent persons to discharge their duties - for example with the manual operation of motor operated points.
We also have operations representatives spread about at busy galas - for example if I am working as the Quorn Movements Inspector, then on behalf of the responsible officer I look after the traffic movements in the area, and also should the crossover points go wrong, it would me, working with the signalman, who operates them, subject to the correct circumstances being in place.