Electrification is very efficient as long as
-mr Putin doesn't start a war (in which case a few well targeted missiles hitting major junctions, national grid feeder transformers and signalling control centres would cripple the network for ages)
Whatever the energy mix, if the signalling control centres get hit the network is going to be out for a significant amount of time. Also this sounds very much like a desperate attempt to justify something which has few advantage.
-there are electrified diversionary routes for all significant lines
If we're arguing that significant lines need diversionary routes of they are wired up, why not if they are diesel (for example Exeter to Plymouth via Tavistock, a second route to Brighton, Alton to Winchester, and many of the other schemes which people could cite)?
Most of the network is too busy to run extra services (for example the GWML hasn't seen extra services because of the landslip at Hook closing a 4 track railway - which by anyone's measure must be a significant line), so most diverted people would have to use the services which used that route already (there are some exceptions like GWR services using the WofE line, but they tend to replace SWR services).
Again, it sounds like a bit of a desperate arguement, as there'd never be enough hydrogen trains to run along diversion routes at the same frequency as they would on their own lines (given by their nature the diversion routes would be longer).
-either all lines are electrified or there is some other means of propulsion not dependent on wires for lesser used lines.
Agreed, however electrification of all lines is going to take time, and beyond the short term it's likely that batteries are likely to give you a more uniform fleet in bridging the gaps in the wires which are short, anything longer (and whilst not ideal) it's likely to be more efficient to use the old diesel trains were have rather than custom build something which could see limited use in (say) 20 years time.
If we had a rolling program of electrification at 30 STM a year per region, then England would be completely weird up in about 35 years. However even at that rate in 20 years time (and bearing in mind there are bimodals trains, like the 38 class 755) there's a good chance that most of the gaps in the wires too long for battery trains could be dealt with by bimodal trains.
To aid in that, arguably the line between Newbury and Taunton could be one of the latter lines to get wires (except for the freight which uses it) to keep using the class 800's.
Hydrogen could play a part, particularly if it was obtained by electrolysis using wind power.
Not a significant part in rail travel, and there's a lot of grey hydrogen which needs to be converted to green before we start using hydrogen for too many other new uses.
That's before we consider how costly it is to move it about.
For example it's more efficient to create hydrogen as an energy store to then use it to create electricity to charge a battery car than it is to use it directly in a car - and charging batteries are less efficient than direct power like OHLE.
Currently our grid isn't in a place where we can afford to be wasting large amounts of electricity on something which isn't that efficient.
There's other energy storage options which could be created fairly cheaply, for example gravity batteries (take a disused mine, put a heavy weight at the bottom linked to a pit wheel and then just it up at times of over supply and then use regenerative braking to get electricity back out as it lowers back down at times of under supply), whilst they require a fair amount of upfront costs, they down degrade (like batteries) or require complex storage systems (like hydrogen).
