Can I enter for an A level and sit the last paper?

I think you really need to talk to your Head of Sixth Form or similar or your school Exams Office to make sure that you have been given appropriate advice for your circumstances.

I called Edexcel and they said yes but they didn't send the written confirmation so now i'm sceptical.
I missed out on a whole a level due to illness. I need a 3rd. I asked if I could use my illness to excuse my absence and to enter for the final RS exam to get a grade. I've been cram studying it.
This is allowed right? I'm scared because the representative said it's fine for me to sit Religious Studies paper 4 but I'm worried because they didn't said the written response as they said they would.

You're entered for a qualification, not individual papers. You must be taking your exams at an exam centre, such as a school or college, so you need to speak to the exams officer there.