I think these two would massively burst the OP's £25b/year budget. The others make sense.
The tunnel to Ireland was reminding people of schemes which had previously been suggested. Generally it's been out into the far too costly pile.
The second tunnel to France was a bit of a fight of fantasy, but was probably something which was an interesting thing to consider if the funding was ongoing and/or shared with France, given the fairly significant value.
SDO would mean that lengthening every platform wouldn't be needed, and given infrastructure congestion at 'pinch points' I would prioritise longer trains over increasing frequency, its also easier to deliver within existing staffing levels.
Indeed, and that was why I said look at increasing frequency otherwise lengthen trains. What I meant by that was, if a line has no pinch points it's likely to be better to run 2*4 coach trains than run a 8 coach train as it would be far more attractive. However if there were pinch points it's likely that increasing frequencies might be much more costly and so lengthening trains could be better.
Likewise if a line has 2tph going to a 4 coach train and an 8 coach train and then to 2*8 coach trains is likely to be the better way forwards, however while that's happening look at what's required to then add in a third train an hour. As that could be quite disruptive if the other services are running on a half hourly basis and you change to a 20 minute frequency for that one service.
However, it could be that having a 15, 30, 15 interval spacing may allow a better connection to another service and so may be better at attracting more people.
It may even be that the extra demand is only over a certain part of the route, so it could be that other services may be added rather than duplicating existing roots. That could be running a train along a branchline and to a different point in the mainline for example to connect that line to a different town/city or could be something else.
Like if the capacity was needed for between Salisbury and Basingstoke and as there's currently no capacity to run more services into London or could be that a new service (by extending the Basingstoke Reading services) is created to provide a Reading Salisbury service.
As such "look at" wasn't a commitment to increase by +1tph but rather a short hand for consider the options whilst having lengthen services as an option.
It's also worth noting that 60% of a 350 is 140 people which is basically a full (seated) class 150 so isn't a lightly used train, as such it's not unreasonable to be considering increasing frequencies. However that value is the trigger point to start the process rather than the point at which new capacity is added, so it may well be that those services get busier before (say) new rolling stock is added to the fleet, which could still take a few years to deliver.