When riders describe their 'seat' readers often come to the idea that there is only one English seat /way of riding. In fact there are numerous 'seats' - each designed to support the rider in different postures:
The flat race jockey sits perched on the saddle; the jump jockey rides with the legs down longer.
The dressage rider sits upright with weight in the saddle and with long stirrups with minimal weight pressing onto the stirrup irons.
The show jumper sits with shortened stirrup leathers and bent legs allowing him to lift up off the saddle and lean forwards and over the horse's neck when the horse is jumping.
The cross country rider carefully adjusts the length of the stirrup leather to allow the rider to rise off the saddle and jump the fences - yet the rider must be supported for the galloping lengths.
The amateur rider adjusts the stirrups to feel comfortable until the muscles in the crotch, thighs and lower back have developed.
An English saddle manufacturer will offer a GP saddle, a dressage saddle, a jumping saddle, a racing saddle, a hunting saddle & an endurance saddle - and mixtures of all cuts. Look at the 'Ideal Saddle Manfr's' web site.
Most established English riders will have more than one saddle for the same horse.
Invariably every horse will have its own GP -general purpose saddle - English saddles cannot readily be swopped from horse to horse for fitting reasons.
The saddle's function is to put the rider in the correct posture on the horse's back
And over the horse's centre of gravity and to keep the rider's weight off the horses spine.
Adjustable stirrup leathers make if possible to allow for the different heights and physiques of a broad mix of riders.
The length of leg a rider needs is to a significant extent determined by the rider's fitness and state of muscle development.
The rider's thigh must be 'rolled' over to allow the leg to fall naturally and make it possible for the rider's heel to lie naturally lower than the toe. In an ideal situation the position of the riders heel and toe determine the length of the stirrup leathers - unless the rider is competing or having a lesson in show jumping when the stirrups should be adjusted short.
It is one thing to adopt the correct sitting position -- what counts is for the rider to be able to maintain it - and that takes muscle development.
Acquiring from the very beginning the correct seat posture is of supreme importance to the novice rider. Sadly too few novices know why.