The important issues in regards to nuclear weapons
About 50% of the population favour Trident renewal. When asked to list their top three, five, ten or (for all I know) hundred issues, it doesn't get a look in. It isn't an electoral issue unless Labour make it one, and when it is made an issue, it is and remains nothing like the winner you appear to think it is. Yes, I have done doorstep canvassing in marginal seats, since you ask.
The strategy you are outlining is the "pile up more votes in such affluent urban seats as we already hold". I don't doubt that CLPs in safe Labour seats are energised by Corbyn, and I don't doubt that there's a chance (not as much of a chance as his supporters think, but a chance) that some Labour MPs in those seats will increase their majorities. For people with relatively safe and secure jobs, like yours or mine, a bit of virtue signalling about nuclear weapons and immigration is great. For people struggling to survive on zero-hours contracts? They have more pressing concerns.
In the meantime, Scotland is completely SNP (ie, Labour can't win a general election without an immense swing) the north of England is a sea of previously safe seats under severe pressure from UKIP which may split the vote enough to let Tories in (look up Ann Black's last Labour NEC report, again nailing the "but UKIP only take votes from the Conservatives" nonsense) and suburban and small town seats all look extremely dubious. On the current boundaries current polling gets the Tories a 150 seat majority; the new boundaries are generally regarded as favouring the Tories, so who knows what it means in 2020?
Yes, Corbyn will give Labour an increased number of votes (I think) and possibly an increased majority in some seats we already hold. Meanwhile, a whole swathe of England north of a line from Bristol to The Wash will haemorrhage votes and seats.
And that's before we start on the ructions that are bound to happen when Corbyn tries to sell unilateral disarmament to the unions: the places that build and maintain the weapons are massively unionised, remember.
www.gmb.org.uk/newsroom/welcome-for-labour-pledge-on-trident
But not, I suspect, the pledge you think they are welcoming.