Despite want has been said above, the technical maximum for most Openreach provided FTTC is 79Mbps down, 19Mbps up.
But as I said earlier, the maximum speed you can get is dependent on the length of the wire from the router to the FTTC cab, the layout of wiring in the house (it it uses a star config, you get too much signal bouce), whether or not you have the ringer wire connected (its a ficking good antenna to pick up interference and add it to your line), how many other users in your same underground (or aerial if rural) cable are heavy users (crosstalk) and the profile applied to you line (basically which compromise you want between speed and stability), and what shit you have plugged in to your line.
Its a legal Ofcom requirement that these services state an "up to xxMbps" speed, and a voluntary code of practice that most ISPs have signed up to to give a reasonable estimate of what you will get. Obviously estimates are challenging due to the nature of the beast, so need a little leeway either way.
I personally get above my estimate on both my lines, one Zen, one BT Business. But then my internal wiring is absolutely bang on geared up for best possible DSL speed.