When should I initiate leaving BT?

You should err on the side of caution , even if it means paying BT a part month payment, you need to give 30 days notice, so that's around 7th October ( already passed that date) , but migrations can be done in 10-14 days, given that starting a migration gives the notice to the provider you are leaving, it's unclear if your old provider only needs 10-14 days notice or sticks to the 30 days.
if you give notice now , by getting Plusnet to start the migration and it takes place before 7th Nov, ( which is quite possible ) , you could be looking at early term charges ,router costs etc.
If you waited until around 28th October, then the changeover should be after 7th Nov, and even if it's a couple of days after that , you pay BT the full month then get a refund of the part month so if for example the changeover was 10th Nov, you only pay for 3 days at the out of contract BT price, the rest of the month is refunded....building in a couple of days 'grace' could be a lot cheaper than changing over a couple of days before the 7th Nov, and given that Plusnet won't really care what date the changeover happens ( it's your problem not theirs ) and the migration date can slip , trying to get it to happen on the exact date your minimum term with BT expires is a bit risky, as is asking Plusnet to migrate now but set the changeover date for the 7th of Nov