The cost of repainting your door could depend on a number of factors. I would estimate between £50 to £150.
Does it need a lot of filling/repair? This could vary the time it takes to complete the job. Minor filling can be sanded and painted on the same day, while extensive repairs take more time to dry and sand.
Are you supplying the materials? Or you just looking for labour costs? Is it going to be water based paint or oil based? Drying times are a lot faster using waterbased products, thus several coats could be applied on the same day.
Do you plan to change the colour of your door? If you wish to change the original colour, it takes longer as you’ll need undercoat and two top coats to achieve a nice, solid finish. While if you stick with the original colour, you can get away patch painting the repaired bits, then giving one top coat overall to finish it.
Hope this helps!