Just to rule out your equipment can you take your router out of the loop by connecting your computer to directly to the modem and run a speed test again.
There would appear to be a problem prior to your property so I doubt anybody on the forum will be able to help.
I have notified the moderators of the forum about your problem. Once they have read this they may be able to help. They are a BT UK based team and if they can help they will reply via this thread asking you to contact them via a link. They are very busy at present so once you have replied to them by the link, it can take 48 hours for them to re-contact you.