We just came back and the booming system was absolutely fine, it was available to book around a week before we went. Have you downloaded the app for the resort you're going to as this is what you use to make your bookings, it's "Butlins" plus the name of which of you're going to.
We managed to get 1-2 activities booked for each day plus one evening show each day and two swimming sessions for the week although I think swimming will be just show up on the day from the 19th onwards. At the shows once the show is about the start they let walk-ins go in, usually for a 7pm show you can walk-in from around 6.50pm, and a fair few people don't bother to turn up so there are almost always walk-ins.
There was no bar service when we were there, table service only, but this might change after the 19th? The table service is done via the B-serve app, you just choose your venue, order your food/drinks, then put in your table number and pay via card, they then bring it all to you.
I really hope they keep the apps as it was handy to book activities and shows in advance as well as have food and drinks brought to the table.
The fairground, arcades, beach, play areas, fountains, pedal bikes, and restaurants didn't need slots booking in advance and in the Pavilion there was a seating area that didn't need to be booked but still had a clear view of the stage so even if you don't have anything booked there are still things to do.
Hope you have a lovely time!