Has anyone tried to book a reservation 45 minutes before departure at the station? Particularly at St Pancras?

I´ am not working for Eurail or Interrail i just share my knowledge here. Please ask in the Community and not via private message as this is the fastest way to get an answer. I prefer English/German/ Czech for my answers. In case of Reservationquestions please share some details like Route, Date, Trainnumber as otherwise we can just provide general advices or answers