The West Coast Main Line is easily at capacity. There are upper limits (as dictated by the government) on maximum fares, but we can still expect fares to decrease at peak times if there is open space on trains. Likewise with off-peak; empty carriages don't pay the bills, and if they're running twice as many trains then either (a) twice as many people will travel, or (b) fares will decrease until they start filling up.

The problem at the moment in off-peak times is that they dare not reduce the price because there are trains that run during the day which fill up, and there's no good way of differentiating fares between the ones which do and the ones which don't.