hi there tara999.
It's fine. You just inform the council by letter of the date when you will be leaving the property and also inform them of the new tenants name and previous address. They will then re-imburse you if you have paid in full upfront, recalculate the tax and bill the new tenants directly from the date you move out.
You must inform the council of what is happening and also inform your tenants what you have done. Provide them with a copy of the letter you have sent with any property ref numbers etc. This covers your back ! If you haven't paid upfront, cancel the DD or await the councils revised bill to you.
Happy Travelling