@irenemacbain Welcome to the forums
As you say you are using Wireless (wi-Fi) there is very little PlusNet or any other ISP for that matter can do, since its outside there control, its totally dependant on issues around you, from walls, neighbours wi-Fi fighting for the very limited bandwidth available for wi-Fi, and getting worse with lots more devices etc being Wi-Fi enabled.
Wired connections are much better and upload/download speed will be based on that for your internet connection not Wi-Fi.
One thing you could try (I am assuming here you have a PlusNet Hub one router) is logon to the router and then split the two Wi-Fi bands 2.4 and 5Ghz so they don't have the same SSID. This can sometimes help a bit.