Hello.
Firstly I have moved your thread to the correct section.
A full paintwork correction is not cheap. £320 for 3 days of work is pretty reasonable, especially if their quality of work is good. What's the company you have gone with?
At the end of the day, you can easily spend 35 hours doing a heavy enhancement / 95% correction detail and it's not only the products that are used you're paying for, it's the cost of the machinery, and more importantly the skill of the person doing the work.
Could you find cheaper, maybe. But depending on who is doing it, you may find that you end up getting a worse result.