i have a fibre250 connection and rarely get the promised speeds on speediest.net initially when used my own apple airport extreme i regularly could get the speeds i expected however using my own router caused issues with my tv package (the wouldd not tune subscription channels i switched over to the smart hub tv works fine but now seemingly getting slower speeds the bt speed check just tells me to contact support and never gives any actual results had the same issue last night and the chat support agent basically said because the ping was <100ms everything was fine?!!
I'm tempted just to with my own router back in and hope tv still works!
I have Ultrafast Fibre 250 too. The best way I find to test the speed is using the BT App or running it from within BT 'My Account' via a web browser. This shows both the speed to the Hub and also the speed to the device. In my case the speed to the Hub recently dropped also after starting to use the Smart Hub 2 but is still very good at 292mbs. A test I also use is the BT Wholesale Test https://speedtest.btwholesale.com/ and if you run further diagnostics it returns an ip profile. My profile shows as 304.41Mbps which has dropped from 319Mbps. I believe this is why my speed has dropped by about 15 Mbps but BT doesn't seem to believe this. I would be interested if you ran the the BT Wholesale test what shows as your ip profile for the Fibre 250 product. Hope this helps.