I am not saying the issue is not sky, always dangerous to claim an isp is innocent.
But sky is not an isp with known congestion problems that are severe enough to affect ADSL throughput.
I would at least for now avoid use of steam downloads whilst diagnosing this because as Andrew said, steam is horrifically designed, its not very network friendly at all. It effectively DDOS's a network from within when a download is running.
You can reduce the negative impact of steam by choosing an asian download server or something, on ADSL with the 30+ threads steam uses it will still easily max your throughput.
Edited by Chrysalis (Thu 07-Jun-18 21:15:04)