One thing's for sure: 820 wasn't withdrawn due to low mileage. It was amongst the last 315s to be overhauled, receiving this work in summer 2019. The society were aware of its withdrawal date several months ahead of time so it was never planned to be a survivor into 2022.
Of the recently scrapped 315s, I know 834 was one which was virtually out of miles. It was due for withdrawal last summer but had a reprieve due to a fault with another.
As a numbers geek, I hope flying machine 839 is there until the very end, ideally even the one preserved, as that has always been a lovely runner.