I notice Ryanair managed to gain an additional 1" on legroom by installing thinner seats. It wouldn't have surprised me if TOCs do something similar in order to fit more seats into a carriage.
I think that they have already started going down that route. Certainly modern seats (like the current GWR HST seating) is more "upright" than the seating it replaced. This allows pretty generous leg room without sacrificing numbers of seats.
The one difference of course between planes and trains is that you can travel on a train without having a seat. This means that the trend towards squeezing more seats in has o compete with the trend towards ripping seats out to give more standing room.