I echo what HarryM1BYT said about speed tests on wifi.
Also some speed testers do give differing results to others, not sure if that is simply down to different browsers and support thereof.
The most common one is
www.speedtest.net - just beware of the adverts it displays.
It is worth putting your phone number into the BT Wholesale DSL checker and seeing what the estimates are for your line -
https://www.dslchecker.bt.com/adsl/adslchecker.welcome