The thing is, if you get 25 Mbps downstream sync rate predicted with FTTC that is better than ADSL2+, and even at the same rate it would be faster because with VDSL you use the PTM protocol whereas for some stupid reason BT does not offer PTM with ADSL2 or ADSL2+ even though the spec supports it, only ATM, which is ~10% slower or ~3% slower
at the same sync rate because it is so bloated and inefficient - a lot of wasteful unnecessary extra bytes being sent, not just your data. Mind you, some modems possibly do not support PTM with ADSL2/2+ so that could be a feeble excuse. Ten percent is a lot of speed to throw away for absolutely no good reason. So you do not want ADSL - with ADSL 21CN, the IP PDU rate is only 88.2% of sync rate according to Kitz, whereas for VDSL2, to quote Kitz again,
BT would appear to be using approx 96.8% of the sync speed to calculate the IP profile for the FTTx products.
Please note that if your DLM profile is set to Retransmission High, then this figure will be nearer 91% to allow for the additional overheads.
(There are other types of overheads too, so further bad news, and further reductions, depending on what data you are including or excluding, because of additional protocol headers. These will always include IPv4 or IPv6 headers, and often TCP headers too, all reducing effective speed still further but these are not dependent on the type of DSL link and are very dependent on the protocols a application uses.)