I don't recall any promise of 100% fibre, and in any event public money usage in this area s nothing to do with Cameron. This issue is full devolved (in terms of the use of public money).
The relevant part of the BDUK money was allocated to the Scottish project and the Scottish parliament is free to set priorities and allocate further public funds as required.
Of course Ofcom provide the regulatory framework across the whole of the UK (so is not devolved) and that does not allow for any pricing mechanism to cross-subsidise broadband provision. Revenues from rural areas go nowhere near covering the increased costs of provision and maintenance and are cross-subsidised from urban areas, but that's just as a functional voice service, not as a broadband service.
Whether Ofcom will be addressing these issues in their new regulatory review, I've no idea, only to say I doubt that they will go so far as to change the regulatory regime to cross-subsidise rural broadband through, say, a levy. (It may be that submissions have been made to them).
In any event, at the moment, the best approach has to be to lobby your political representatives. Given that it's difficult to see how any commercial company will consider it financially viable, then it would require either more public subsidy, self-help or an explicit change to the regulatory regime to allow for the required cross-subsidisation.