You can do your return as late as 31 October but why would you? The sooner you do it, the sooner you can use the money refunded to you. Especially if you want compounding effects.
I email my tracker to my agent on 1st July, she usually has it submitted within a week, and my refund is often done within another 2 weeks. The refund goes straight into my WISE account and then it’s time to decide what to do with it.
Previously I have (feel free to copy):