Something to keep in mind a lot of routers have moved to fixed antennas even some that have SMA connections on the first revision which makes it a pain for adding in an external one.
Despite a quoted range of around 30-35 down and 5-7 up for FTTC for where I'm living at the moment the line only does around 24 down and 1.1 up due to "bridge taps" and probably line conditions in general - 4G has been a god send for me since moving. Need to get an engineer out still to sort the line but having some account issues as BT as usual is a mess when something actually goes wrong. So could be something similar in this case as well.
So OFCOM regulations state that a provider must make every and all effort to get the speeds you have been quoted and if not they have to let you leave if the speeds can't be improved in 30 days.