Hi
As far as your specific question is concerned, I think it's reasonable to say that consultants are generally regarded as being self-employed.
This is supported by the information under 'checking their employment rights' in the link below.
Employment status
If we accept that this is so, and assume that the consultant is a French tax resident, then their profits would be taxable in France only unless they have a UK permanent establishment (Article 7 UK/France DTA).
It seems that the question being asked of us is the position from the point of view of the company.
As we would only respond on this forum board to questions relating to personal tax, we have forwarded your query to the Business Tax Customer Forum which might be better placed to provide an answer.
Thank you