we are looking at train prices from London to Canterbury in June. There are direct trains that take about an hour but prices vary significantly. For example one leaves St Pancras at 9:12am and costs £43.70; it is also labeled in a pulldown as an "Anytime Day Single". A train leaving an hour later at 10:12am costs £15.20 and is labeled Advance (Standard Class).
So if we wanted the 10:12 train do we need to buy it now/soon? I'm not clear if the lower price is because it's off-peak travel and we could get that rate even on our day of travel, or because a limited number of cheaper fares are sold and they might have already sold out for the 9:12am train.