I have been using Barry Swain holistic tree saddles and I'm a convert. I have a HM/BS dressage and a KH/BS GP.
You could always buy a second hand on Ebay and have it altered by Better Saddles.
I agree with Chapsi - these are beautiful saddles. I had a HM dressage one which my cob outgrew and I sold on and then a BS/KH GP with an Owen half panel.
When I first had the GP - which was made to measure - it was brilliant - didn't slide sideways at all - but then my mare had colic surgery and changed shape and the saddle hasn't fitted since. I've still got that saddle but it's an 18" and it's wider than the XW Thorowgood, so far too big for your horse.