What a stupid uneducated idea!
It's just not possible, I'm not sure if I should explain the reasons.
For one who is gonna want to be a standby driver on the odd chance theres a strike? The last time we striked was over 10 years ago!
Who's going to train them? What are they going to do in the mean time? How do they keep their competency up? How could you do all of that with the amount of standby drivers you'd need? Even for a skeleton service less than a Sunday service you'd need at least 20 drivers a depot and my region has around 10 depots (may be missing one).
Even If they end up driving regularly to keep competency up they'll soon end up joining the union anyway, when they realise they need the protection because of the nature of the job and conditions.
You can't just turn up once in so many years and drive a train up and down.
It won't be the unions who wouldn't primarily like it, it would be the office of road and rail, the rail safety and standards board among others
I'd like to see your reasons for thinking this would work.
Driverless trains are long way off, I'll be in a job till I retire and I'm 30.