I'm not sure if this is useful here, to keep track of how long things are taking, but I just got my biometrics invitation letter today.
So, confirmed receipt of my application in Durham: 20 August 2018
30 August: the date on a letter I received acknowledging my application and saying they'd decide within 8 weeks unless something out of their control came up
14 September: the date on the letter I received today, which I can go get my biometrics with.