BT is the UK's largest residential ISP, running on the Openreach network. Plans range from legacy ADSL to 1.6 Gbps Full Fibre. The biggest factor in your real-world speed is which Openreach product you actually have.
Distance from the cabinet matters. Run a wired test directly into the Smart Hub. If still low, raise a fault, Openreach can re-jumper your line at the cabinet.
Switch to Smart Hub 2 or add a Wi-Fi 6 mesh (Plume, Eero, Deco). Wi-Fi often loses 30-40% on FTTP plans above 300 Mbps.
Usually upstream congestion at the BT exchange in evening peak. Test at noon and 9 PM, if the gap is large, you've hit it.