No, I think the railway should employ enough staff that the normal day to day operation is covered by staff within their contracted standard hours. That includes normal expected levels of sickness and the "latent" level of day to day disruption that occurs.
Overtime should, in any business, be an exception rather than a rule, as the day to day need for it is just poor planning.
If, say, a Pendolino takes the wires down in the evening peak in the Euston throat, you're probably going to need some overtime, of course.
Its more complex than that. Drivers can be off/not available. (Retired staff, depot transfers, staff who have exit the TOC, operational or long term sick.)
Training a driver is expensive to any toc as is finding new drivers. A lot of these training courses are paid for by the DFT. Tocs rarely dip into their own pockets to recruit new drivers. In addition, at depots you can have local agreements which also makes things more complicated.
For example one depot can throw a Sunday in by Wednesday of that week, whereas a depot 40miles away has to apply for the day off or mutually agree a swap with someone at their depot who is off.
Plus you have complications when it comes to route signing. The demand for that driver goes up if only that one particular link signs the route or diversion. Training a whole depot on that wouldn’t work, because of competency, driver availability to go and route learn. With that said overtime is available for instance when new traction is introduced or amendments to that links route card. Where drivers are released, other routes/traction becomes uncovered. This is when overtime is rewarded like a big win at a casino. It’s very difficult for a toc to keep up as it’s an industry that keeps changing every 6months. DFT demands it, TOC has to deliver.
To the OPs question, I would also add a cut down on depot numbers would come first. For instance if depots total heads is 300 and they currently have 200 drivers at that depot, then the cuts would come down to the quotas. So 300 to 250. Also redundancy would be packaged to nearing retiring staff.
Nothing is guaranteed, but it certainly is stable so far. (Especially in the driver grade)