The Graph on this site may help you -
I would recommend that you try to improve the line by doing Quiet Line Tests etc whilst the line is still on ADSL of some variety.
You should also try to pin down the length (and route) of the D-side cable from the PCP to the house, to a reasonable degree of accuracy, as can be seen by the ski-slope in that graph.
Keep in mind that effectively, the above critical D-Side length, will actually increase if upgrading to FTTC/VDSL is carried out.
This additional length is the D-Side link from the FTTC to PCP.
This extra length can vary from being very short; say about 5 Metres when the FTTC and the PCP are close to one another, through in my case 50 Metres; and I have seen suggestions of longer.
As VDSL works in higher frequency bands than ADSL, the losses per unit length of cable/phone line from the FTTC via PCP to house.
I have not tried working it out in detail; but let's say that the losses are 5 times as great, hence the criticality of that length.
Depending on the precise circumstvances, staying on ADSL with a cleaned-line may give better results than "upgrading" to VDSL.
Also within VDSL, it may be better to subscribe to a 40/10 package, rather than an 80/20.
But it is difficult to accurately forecast the working results.
Ironically, "my" FTTC is only 10 Metres from my front door; and the PCP about 40 Metres away.
Yet the D-side from the PCP to my house, wanders about 250 Metres on a green-field site, giving a length of 300 Metres for VDSL purposes - and it may be longer!
That Line Attenuation is about 13.5 db for VDSL.
Compared to 24 db for the previous ADSL, coming from the Exchange; about 1,284 Metres.