Your FTTC (VDSL) connection speed will depend on the length and quality of the “twisted pair” (copper and/or aluminium wire) connecting you to whichever BT street cabinet services you. Note that this cable does not necessarily take the shortest route. Add into the mix the unfortunate fact that these cables and connections are generally in poor condition around here, and subject to external interference from power lines, electric fences and so on, and you can see that it is impossible for anyone to answer this question with any accuracy.
The maximum connection speed for a property right next to the cabinet is now around 76Mbps download, depending on the service selected. The maximum download speed could eventually be pushed up to around 100Mbps using this technology. If your house is close to your cabinet then you can expect a VDSL FTTC connection to achieve better than 30Mbps throughput on the standard services, and over 70Mbps on the premium services. Speeds then tail off further out and are highly dependant on line quality. Some with very long or poor lines will see no improvement. If your line is very poor, then it may be impossible to get any FTTC service, in which case you may be stuck with whatever you have currently, for the time being. Surveys of existing FTTC connections suggest an average of around 30Mbps, but these results must primarily be from urban customers; read more here and see latest survey results here.
BT do try to provide an estimated speed based on their database, and indicate whether the service will be available on the specified line. In some cases this may be very inaccurate, but it usually gives a pretty good indication. Click here for the BT checker, but please read our FTTC guide before ordering. Also bear in mind that the wiring and equipment in your property can also slow things down, but that’s the subject of another question.