i cant find a airline that goes from the uk....

To best of my knowledge no airlines, at the moment, flies direct to Djerba in winter time. You would probably have to to put holiday together yourself and fly with B.A. or Tunisair to Tunis then domestic flight down from there. You probably have to stay in Tunis overnight beginning or end of holiday depending on flight schedules.

There is an independent company that would arrange this holiday for you

http://www.tunisiafirst.co.uk/

Obviously as a tailor made holiday may be rather pricey.