I wish they weren't as 'ten a penny' as they currently are. Perhaps restricted to groups of 3 or more, Family/Senior/Disabled Railcards (or other proof of age/Disability), etc.
Perhaps a charge of £5 on everybody else.
What would also help of course would be to have trains consistently of sufficient capacity in the first place so people didn't feel the need to reserve seats.
Notwithstanding, if implemented effectively, seat reservations can have the effect of helping spread passengers along the length of the train, evening loadings and helping dwell times. However, the UK always seems rubbish at inplemebnting these - either clustering all reservations together in the same carriage on an otherwise empty train, or not giving 'Composition des Trains' information on the platform when boarding, so the spreading effect is lost.
With technology, the alternative is to go the whole hog and make long distance trains compulsory reservation. That does not necessarily mean losing the walk-up railway, but selecting your seat(s) on an App when you know when you want to travel. Spontineity being preserved by making quotas of seats unreservable until 10/20/30 etc. minutes before departure.