Can't you upgrade to fiber yet?
Also, what service are you on? 8Mb or 24Mb?
When I was on 8Mb with TalkTalk I received anywhere between 3-5Mb, then upgraded to 24Mb, and received around 6Mb.
Since making the switch to 38Mb FTTC from TalkTalk, I now get a rock solid line of 37.xMb all day every day!