TOTB Sunday you need to pre-enter. Generally this is via car forums/clubs where they try to get ten entries from each club
But there are private/invite entries available. www.totb.co.uk

The Saturday is a RWYB and open to everyone. Not sure if all places are filled there or if you can enter on the day. I suspect you would be able to enter on the day.
And some of the fastest entries on Saturday get a "wild card" free entry to TOTB on the Sunday.
or something along those lines.