is a document that defines what frequencies can be used in the Access network (ie all the copper lines) in the UK. It has to be met by anyone that wants to connect into BT's network (or KCOM's network, with a different version of the ANFP).
The body that defines the ANFP is the NICC
, which sets the standards, but is a committee that works under the auspices of Ofcom.
The ANFP defines that the frequencies used by VDSL2 (ie the variant of DSL used in FTTC) can only be generated from the cabinet in the street, and not the exchange building itself. It doesn't specify why this is the case, but it is undoubtedly to prevent problems in interworking with the other equipment located in the exchange building, or to prevent interference for customers who use ADSL from the exchange (which may be BT or an LLU such as Sky).
The ANFP used to allow up to 7MHz from the cabinets, but has recently changed to allow up to 17MHz. As someone else has mentioned, it is the BT R&D department that will have a big input into the committee's decisions about what is workable within the access network.
So... The restriction in FTTC comes from Ofcom, delegated to a sub-committee, under guidance from BT engineers.
The restriction in FTTP is ... probably ... a mix of financial restriction, plus a general wariness because the techniques for deployment of FTTP are not as mature as those for deployment of FTTC.
IMO the best that people on Direct-Exchange lines can hope for is that BT get a good take-up of their FTTx products, which makes them reconsider deployment out to more & more of the "not-quite-standard" subscribers.