The other slight issue that BT Wholesale run the ADSL2+ DSLAM's

and

VDSL2 ones are run by Openreach in the UK, and the need to support VULA etc

that's now, and could change, but in any case Openreach operate VDSL under some sort of concession from OFCOM and there is no reason why BTW cannot offer VDSL using the same wholesaling and EOI criteria as ADSL, surely.

Yeah, but I can't see any technical reason why this wouldn't work.

If anything, cabinets should be just as bad for crosstalk - if you're on a 3km line with a 200m dside competing with a VDSL cabinet going at full pelt the crosstalk is going to be just as bad (or worse) if the frequencies overlap (hence the masking you mention).

I don't see why the 'cabinet' being in the exchange makes any difference at all - I would be interested to know if there is a genuine RF reason that I'm missing here.

To me, it seems like archaic rules...

There are power masks so that a VDSL cabinet does not go at full pelt even out in the street

the "rules" are by omission rather than design. Nobody has found it necessary to submit a plan for exchange based VDSL2 to NICC. Length of internal exchange wiring, length of EO lines and lack of sufficient EO numbers have been cited as reasons not to bother.

There is no ADSL2+ much beyond 2 MHz so the sort of masks used in cabinets (which are not power reduced beyond 2MHz) could be used in the exchange - unless there are non-xDSL services that would be impacted. DSLAMs do both in the same chassis so there's no inherent conflict.

Part D of the ANFP says "It is for future study whether for these EO lines, an enhanced PSD Mask specification can be produced and agreed that would allow the use of higher frequencies than those allowed in Part A taking account that there is no SLCP for connection of transmission equipment."