Re: It's not all twisted pair…
I know that the distribution cable isn't as highly specified as highly as modern data cables of course. It's a fairly loose twist. It is therefore, of course, not as resistant to EMI (and, therefore will also "leak" more signal), but this has to be kept in perspective - if it can be considered and antenna, then it's an incredibly inefficient one. The biggest problem is with cross-talk within multi-pair cables, and I've yet to see any evidence that external EMI is a significant issue to other communications with existing DSL (which, after all, goes up to 30Mhz). As people will know, lower frequency radio waves (in general), travel further with less attenuation than higher, so I'd expect frequencies up to 100Mhz to propogate even less.
As far as the sub-loop is concerned (the bit from the "green cabinets" to the household), then the ANFP-S for VDSL (only currently carried over the sub-loop) certainly refers to this is a twisted pair network.
"it is applicable to all sub-loops in the BT access network provided using unscreened twisted metallic pairs." It goes on to specifically exclude fibre provision (which is not relevant of course). Maybe there are non-twisted sub-loops (but I'd be surprised), then they aren't going to be suitable for vdsl (and, of course, g.fast) too.
All BT local access loop cables have a specification as regards the balance to earth, mutual capacitance and so on. The main standards are CW 1128 & CW 1128/1179.
There are other suppliers with single and dual pair cables to the same spec.
There's also a well-known spec for extension cabling (and there are external and armored versions available). This is also twisted pair and is known as CW1308, albeit I don't think it is used as part of the sub-loop itself.
The standard for "drop wire" is CW1411/CW1417, and whilst these may not be twisted par as such, they still included relevant specifications for balance to earth, mutual capacitance and so on.
In general, the cables are designed to have considerable rejection of interference and, by dint of this, make very poor antennas.
From what I can find, egress interference from VDSL signals to amateur bands is not usually much of an issue due to relatively low VDSL power densities. Indeed the reverse ingress interference is more of an issue due to the much higher power densities of amateur transmitters.
In general, I can't find much in the way of actual, rather than theoretical, egress interference from existing DSL services despite the fact they overlap a considerable number of bands up to 30Mhz. In other words, is this really a problem in practice (unlike powerline transmission).