and also the idea of the occasional places and waiting for an offer, this is just all new to me to be honest. And I was wondering what happens if you accept an offer from the school and then change your mind because another offer opened up, what do you do?
In general- the London day schools all tend to make their offers in the week before Feb half term. And they all have virtually the same acceptance deadline, from memory early March. Schools will make firm offers and also some “waiting list” offers for kids who nearly made the cut.
Life can be stressful for those who have been given a “waiting list” offer in Feb. Generally they need to take the best firm offer they have at the March deadline (and pay the deposit for it), and then wait and see if they get something they prefer after that. Schools will go to their waiting list when others have turned firm offers down.
As for the deposit, some schools will refund the deposit if you rescind your acceptance early enough, others will not.
Prep school heads can be very helpful at this stage, for example if you are on a WL for your preferred school and will definitely accept it if it turns into a firm offer, then the prep schools can pass this information on and this helps make it more likely that the school will offer to you if they can (since they all prefer making offers they know will be accepted).
Not all schools use waiting lists and even when they do, they may never end up giving offers to people on the WL - it all depends on how many accepted their first round offers in Feb.
Occasional places is a different thing and is moving during year 7 or 8, this depends on someone leaving a place opening up, usually you would need to reapply for these.