Where photographic images are used merely as a starting point,
and you do not intend that the final image closely resemble the essential features of the source photographs, there is less likely to be a breach of copyright. The legal test will be to place the original and copied images side by side, in order for a layman - i.e. not an artist or someone trained in the arts - to judge whether it is obvious that the two are visually connected. If they are, a breach would be likely.
Where an artist is working from composite images, i.e. if an artist works from several images simultaneously for inspiration, it is possible (though not certain) that he or she does not copy a ‘substantial part' of any of them in the creative fusion of these images. However, the test remains qualitative, not quantitative, and so if a key part or parts of an image are used, even if quantitatively small, this may be a ‘substantial part'. Technically, the work will infringe copyright in the photographic works you are sampling if you are scanning or reproducing them in any way. If you are simply collaging the raw material it is less likely you will breach copyright.
Assuming that an image is copyright protected, UK copyright law nevertheless permits elements of such an image to be appropriated and used in new artworks on the basis that the new artwork is not substantially derived from someone else's earlier copyright work. In other words, there is no infringement of copyright if the new artwork does not appropriate all or most of someone else's earlier copyright work, but uses inconsequential elements of it together with other images created by the new author; a collage of other people's copyright images might be an example. Having said all that, the particular circumstances of each case will have to be looked at by the courts to finally decide the matter - there is no guarantee that a jury would agree with you on what constitutes a ‘substantial part'.
Erring on the side of caution and approaching the owner of the two-dimensional image to obtain a licence for the use of the image would be the better approach as it would avoid triggering a later dispute over potential copyright infringement. The downside of approaching the owner of the work could be a tacit admission that the image is copyright protected when this is debatable. This might require some skilful negotiation and a specialist art lawyer should be approached for this.
The issue of whether or not new photographic images of an older two-dimensional image can acquire copyright protection, on the basis that it is not a copy but an original work, has yet to be decided by the UK courts; however the US courts have decided the issue.