It's great to see you have saved money and pre booked your activities.
You wont need to pre book your time slots as these will be allocated for you and you will see your times in the app in the wallet section. However, if you find these times are unsuitable you can go to any booking points once on the resort and change the times, subject to availability.
Don't forget to download the app for the resort you are staying at. Just search Butlins + resort on the App Store or Google Play and log in with your Butlin's account details.