What irks me is when you pre-book, the train is delayed then your seat reservation isn't worth diddly squat.
I haven't thought this through but personally I don't think you should be able to reserve seats on trains, it just causes a lot of hassle in my experience:
-On busy trains (the most likely ones you 'need' to book for), often you can't even reach your seat without barging past all the standing people.
-Even when people can reach their booked seat, they then have to evict the squatter which sometimes results in a debate "but but but I've booked a seat too and I can't get to it / where do you want me to go I can't even get out into the aisle / **** off" or they feel guilty because a pregnant/disabled/elderly person is sat in it
-On trains where reservations are simply bits of paper stuck in the top of the seat, it results in endless streams of people checking the reservations, leaning over each other on crowded trains etc
-In any event I've seen unscrupulous characters just removing a reservation from seats they haven't booked (judging by the fact they got on at a different station than the starting point of the reservation)
-If the train isn't busy, then almost by definition there is little need for a booked seat, it is just a waste of time/resources
Anyone who ever gets the 17:24 Bristol > Portsmouth train should know where I'm coming from with much of the above, as it is always, without fail, standing room only - I've literally never been on it when there has been space for everyone to sit down
Anyway going back on topic I'm usually fairly generous and will give up my seat if I think there is a genuine reason why I should (physical condition whether it be pregnancy, age, disability or whatever). Typically if I'm stood waiting for free seats at the next stop I'll also give first dibs to women.