Suits the Tories fine, its a major weak spot for Corbyn & they'll want it dominating the headlines as long as possible. I wouldn't be surprised if it was a planned move, strikes me as classic Crosby. If we have a week of long, handwringing Guardian articles and short nasty Daily Mail pieces articulating both sides of the issue, that keeps the focus exactly where they want it to be. It may backfire on the Tories of course, but I suspect most people don't appreciate the difference between a first & a retaliatory strike anyway, so the Tories just get lots of quotes of Corbyn saying "I would never use Trident" that they can spin. The Tories love to fight dirty.
For Labour's part, their failure to lock down the Trident issue is baffling. Everyone knows its a vote killer, so Corbyn's team needed to get it right from the off. Even if they campaigned on dismantling it, at least that a strong and clear position. Corbyn's non-answer on Marr means the position of the leader not supporting it despite it being party policy continues. That not only makes him look weak & the party look divided, but it allows both his pro- and anti-nuclear opponents to paint him as they want to.