We moved house two months ago to a rural new build property 3/4 mile from the cabinet, which is another 4-5 miles from the exchange.
BT initially offered us a guarantee of 0mbps !!
So, I ordered a 5G GigaCube mobile router from Vodafone with Unlimited Data (you can get them Mikki
)
And I sent it back 2 days later.
Speed was worse than tethering my mobile, sometimes less than ONE mbps, and it constantly had DNS issues with websites, as well as streaming from Sky or Netflix.
So I tried getting BT out again who finished installing a new line to us just last week and we are now consistently getting around 35mbps.
As HP said, nothing you can really do to improve the speed at source, but do make sure the Master Socket is the latest version (Mk3 I think) which will have two sockets, one for your phone, one for your router, this will give a less noisy signal than a filter on an old style master socket.
BT fitted ours with the old style, I promptly swapped it.