I sympathise.
1. One issue is the rate your connection to the exchange can go at. Does "Fibre extra" mean up to 80/20 or up to 40/10 or something else?
Do you have some way of determining you "sync rate", or "line rate", or whatever Plusnet call it, like from your modem/router to Openreach's network?
2. What do you mean by "getting only 37 mb"? Does that mean by a speedtest, from a particular computer? All computers? By Wi-fi or ethernet? I appreciate you say you've singled it out. So, cutting out a lot of other questions,...
3. It can be very difficult to tell what in your house is using all the bandwidth available to you. You may be able to tell by doing a lot of geeky things with the equipment your house, but, bottom-line, the first thing I would advise, if you are sure the issue must be some computer/tablet/fire-stick/tablet/phone, is just to try turning things off, for a few hours, one after another, using any means available to you to see if that improves things. I have been through a similar exercise. I am retired, and have plenty of time. It is quite astonishing how much internet traffic is caused by a tiny dongle stuck into a HDMI port in you telly. Or some innocent looking tablet. (I might mention "tcpdump", for your googling pleasure.) You can spend a long time and brain-cells getting to the bottom of it. In my case, after several weeks investigations into things I never wanted to know about, turned out to be several devices in my house overloading an elderly router. Solved by spending money on a new one. Your problem may be completely different.
So, not necessarily an "outside the house" (Plusnet/Openreach), or even "inside the house" (your gear) problem. But either. Or both. If it is inside the house, then depending on your gadgetry, you have to expend quite some energy, time, and brain-cells learning how to look into each gadget's bandwidth consumption, depending on it's operating system/hardware and so on. Google is your friend. Or pay someone you trust, with beer or folding-money to advise you.
