The 2 metre rule relates to whether or not express planning permission is required. If it is required, then an application should be made and will be assessed on the basis of whether or not significant harm will arise from the fence's presence. Of course, the need for a planning application will involve cost and delay for the person who wants to build the fence, and may result in a refusal. It's only fair that you as a neighbour should point this out to them and suggest that they should follow the proper procedure by contacting the local planning authority for advice. If you feel strongly that the fence would cause you significant loss of residential amenity, then you should fight your corner; if not, then let them go ahead at their own risk.