Attached is a plot from our SamKnows box on our home broadband showing our upload speeds for the last 12 months.
For Jul 2017 to Feb 2018 it was stable around 1.8Mb/s up.
From March 2018 to May 2018 it was consistently over 2.0Mb/s.
From mid-May 2018 it jumped over 3.0Mb/s for 3-4 weeks.
Then around the 13th June 2018 it then dropped below 1.0Mb/s and has then remained there.
Nothing has changed on my setup, it's a Draytek 2860 VDSL/ADSL router with a tweaked SNR on the downsteam giving us an average of 22Mb/s. We are on a very long piece of copper (1.8km) to the street cab hence our relatively low download speed. Without tweaking SNR it's around 17Mb/s. The router is on a UPS, so other than router firmware updates it never goes off, so it's a stable connection. This is always consistent.
Openreach wouldn't investigate these irregularities in upload speeds and it will still be within their 'expected' parameters, but to me as a person working in University networking and IT it looks like a configuration change on the routing equipment within the exchange making the speed profile change like this.