There is no problem with creating a sculpture which depicts a person, whether alive or dead, either from the point of view of copyright or the person's privacy rights. The situation is much the same as creating a portrait with paint or pencil. However assuming that you are not doing the sculpture from life, with the cooperation of the sitter, you need to be careful not to infringe the copyright in any source material such as a photograph of the subject, unless of course you took the photograph. Section 17(3)
of the Copyright Designs and Patents Act 1988 specifically mentions that, in the context of infringement, copying an artistic work such as a photograph, includes the making of a copy in three dimensions of a two-dimensional work. Making a sculture digitally increases the chance of directly copying the source in a way which could lead to an infringement claim, especially where you rely on the software to create any unseen detail, rather than creating this detail from your own imagination. The problem with digital scanning is that it does exactly what you don't want, that is to say, it makes a faithful reproduction of the source image.
So in the case of a well-known person such as Winston Churchill, you should find as many different photographs of him as you can and base your sculpture on an amalgam of all of them, so that you avoid the danger of copying the specific characteristics of a single photograph. Where you can only find a single image of a less well known subject to work from, I would suggest you study the photograph very carefully beforehand to create an accurate mental image, or make an intermediate sketch, and then put the photograph away while you manually sculpt any detail in, say, the pose or expression on the face. That way you avoid the chance of unconsciously copying the key features which make the photograph unique. This is not the same as avoiding the physical features of the subject which make him or her distinctive, and thus recognisible. So for example, the inclusion of a cigar in a sculpture of Churchill might be appropriate since this is a feature we tend to associate with him. However try to avoid placing the cigar in the same position as it appears in a single photograph. Much the same approach should be applied to digitally creating a sculpture. Don't just scan a single image and then allow the software to render this in 3D. The absence of significant creativity on your part will increase the risk of copying which infringes. If you can find two or three images which are sufficiently similar to allow the software to blend them together, this reduces the chance of copying which infringes. It would be even better to first make a physical intermediate sketch in 2D which you then scan and work on digitally.
It might be worth making a note of the actual source material you use for any given subject, so that if at some later stage it is suggested that you copied a specific photograph, you can show that, in fact, you took your inspiration from a range of images. I imagine that in order to find the detail necessary to sculp details which are not visible in a single photograph, such as the back of the head, you would try to use a range of photographs from different angles anyway. In reality you don't face very much risk of infringement provided that you use as much of your own imagination and personal style as possible when creating the sculpture.