Thats not quite true as I'm aware. The Documents apply to all building work, with some exemptions.
Part M would not apply to an alteration or extension to an existing dwelling. However, as I and others have already suggested a major refurbishment of a dwelling, e.g. completely renovated for example, requiring structural change, then Part M and others may be applied. There are 'limits on application'.
To say it applies only to new builds, is not quite correct IMO. Sorry if I'm being picky.
I think we are all agreed that in this instance a rewire does not constitute material alteration. But HHD will need to discuss this with his scheme. Its his assessment with them.