Very few ex LMS loco's got used on the WR and ever fewer got used on the SR, yes there were visiting Loco's of ex LMS origin but that doesn't create the feel, what don't *you* understand?
I really can't see why you can't see that, one can't model an area (realistically) if the *native* loco's and coaching stock are not available, for example, one of the reasons that the Kings Cross (inner/outer) suburban area or the LNER can't be modelled (without kit / scratch building) effectively during the BR steam era is because the widely used L1 tank locos and Quadarc coaching stock is not available, even though just about all the major express, mixed traffic locos and mainline coaching stock is. I would love Bachmann or Hornby to release a L1 tank but I'm not going to hold my breath, and as for the transition era and a Baby Deltic....
I suspect you are finding argument to disguise your dislike of the SR so you don't have to admit to it, no problems, but please don't attempt to impose your likes / dislikes on others - Oh and don't assume that just because you don't sell many SR locos please don't assume traders south and inline with the Themes don't! ;~)