Well scrapping Son of Trident would save UK£30-40 billion in capital costs, UK£2.5 billion annually in running costs and UK£15 billion in disposal costs.
That's a lot of UK£350 million weeks...
Except it was Labour Party policy (only voted for in September last year) and he was not in a position to change it until the next conference.
If he did then manage to change it at the next conference, since the next election is likely 2020, all that money will have been at work for the best part of 4 years and scrapping it would have saved bugger all.
It was a fight that simply didn't need to be fought.
And at a time when he should have been making hay over Tory policies.