No, you don’t have to plan in advance. If you want to travel on a popular routes that requires a reservation, like the Eurostar between UK and mainland Europe, it is good to make a reservation well in advance. Otherwise you might be unable to travel on the day you wish.

Thanks!! So it’s ok if I don’t complete anything ? I just put London, the number of days and that’s it? 

I don't understand what you mean by "put London". You don't have to say how many days you will spend in a specific town anywhere.

I mean not to write all the steps of the travel. There’s a map when i want to book and I wonder if I have to list every city or if it’s not necessary, I can’t find some of them (Cambridge for example). 

Ok. You only add the trips that you know that you will make just befor you will make the trip. If you can't find the station in the app you will have to add the station manually. 

The map doesn't show all stations. Have you looked for the stations in the app?

Ok so not today though…  ? thank you so much for your answers and your patience!