My mum has BT. She lives in a small town in the Scottish Borders, a town that does not even have a Bank in it. It is ten miles from the nearest larger town and about 50 miles from the nearest city, but her connection shows an 80Mbps down and a 20 up, while I live in Kent County Town of Maidstone, which is pretty big, and has many banks, but I am stuck at a truly **bleep** 34Mbps down and 8 UP?
The main "issue" will be distance to the Fibre cabinet. The longer the distance the slower the speed. There are other factors such as crosstalk (other users taking fibre)
this graph shows how speed drops with distance from street cabinet