The Hub One is just a re-badged version of the Homehub 5 type A, and given that I'm a bit surprised you're finding the WiFi quite so bad at such a short distance.
I assume you have tried changing the WiFi channel number to see if that helps at all?
Also, you have a choice of 2.4Ghz and 5Ghz wireless on that router - you may find that the 2.4Ghz, although slower, gives better range than the 5Ghz. On the Homehubs both the 2.4G and 5G WiFi have the same SSID - it's worth changing that so you can select which you are using (assuming Plusnet have done the same)..
You could also consider using a WiFi repeater (I won't suggest HomePlugs as I think they are evil things
) at a point between the router and the upstairs room to assist with the WiFi range.
With respect to the blue light, on the Homehub there is a option to dim it - I don't know if Plusnet have given you this option too.