Longer, less frequent trains, which will not only be cheaper to operate but also be more punctual and reliable. I'm not suggesting silly-low frequencies, but in the North West the pre-1998 timetable would seem a reasonable guide to what is needed - that by and large operated reliably through Castlefield. Similarly with XC - double up all the Voyagers and halve the timetable give or take busy bits.
In the South East, an all-week (possibly except Sunday) timetable - the commuter demand won't fully return so there will be no need to run peak extras any more, just lengthen the odd diagram, maybe requiring about 5-6 additional units over the base for the south WCML. Not having the London commuter peak is likely to be the biggest saving imaginable I would say.
End competition, which where it exists causes duplication of resources. An example, though it does to some extent take demand from coaches and cars, the Trent Valley local service would be fine as a 4-car set if it didn't have fares deliberately designed to take passengers from Avanti services, which are often not busy as a result.
End Delay Repay. It's become an industry in itself and takes up so much time to deliver it that it prevents a proper responsive customer services operation. Similar for delay attribution as someone else already said.
More DOO (sorry, but it had to go in; it would be a big saving if all urban routes could be switched over, though we would need to deal with things like accessibility - perhaps one-off spending on low floor trains for the long-term saving?). A long term view towards unsupervised ATO where feasible, i.e. "guard only operation".
Closure of ticket offices, supported by the roll-out of contactless payment in urban areas and other means of obtaining tickets, and simplification of the fare structure to abolish things like excesses and other complexities you can't get online or at a TVM. With single fare pricing as being trialled on the ECML you can do refund-and-replace instead of excesses. Also there needs to be an online method of obtaining a reservation without a ticket sale. Furthermore all railway sales points need to be able to deal with all bookings. I would probably retain customer service centres in major stations (probably about 20 of these at most nationally) - all other ticket offices would go.
Technology improvements on rural branch lines (no, not ETCS, something cheaper) so they can be operated using literally just a unit and a driver (and guard if you must). That brings the cost down to a similar level as a bus service.
Beyond that I think you get onto the rather more controversial issue of actual route closures.
Some of those obviously more controversial than others.