Our family of of 5 will be staying in London 9 days, and making a day trip to Cambridge. We are debating if we need to book the train tickets in advance. We research on the train's website and understand price is much better if after rush hour. It also seems the more tickets we buy, the cheaper per ticket. We are just not sure if there is advantage or need to purchase ahead - e.g. train is full, cheaper online and/or advance purchase.
Any advise will be appreciated!