Comfortable, lightweight, great heat transfer through the serge panels, monoflap, Freedom (hoop) tree to fit the type of back you're describing, dressage balance and stirrup bar position, and attractive: Black Country Equinox.
Thanks, y'all! I'm really appreciative of your suggestions. And thanks to Kitt for the excellent advice re: ground driving for a couple of months. Of course it makes total sense, and should have been the obvious first step from the git-go, but as I mentioned previously, I am a moron.
Dreadful Acres: the chronicle of my extraordinary unsuitability to country life
Not a moron - just looking forward! The Equinox is a great choice if your horse needs a hoop tree (though it can be made on pretty much any tree that BC offers), and the serge panels are a great option for breathability and a cool back - and most UK saddle makers offer that option.
And you're very welcome. But trust me about the ground work ...
I have both a Fhoenix and a Stonewall. Love them both. The Fhoenix would be a better choice as a partial tree saddle it will allow your horse to condition and still have a good fit. I've ridden a lot of miles in that Fhoenix on several different horses without making them sore. My Fhoenix is a dressage model and the flap is a bit too straight for my comfort in a long distance ride so I got an Abetta a year or so ago. I have a love hate relationship with that saddle and the worksmanship is just not that good. I certainly don't find it that comfortable and have tried a lot of different seat covers. I have never gotten sore from the Fhoenix and neither have any of my horses.
The Stonewall is a semi custom saddle for my very narrow appy Spanish Mustang gelding and I love it also. I know that the owner of that company can custom fit any horse and she has a lot of used saddles she can often match up for someone's needs. I truly enjoy working with Jackie and would recommend her highly.
you don't have to get a special endurance saddle, especially if all you are planning to do is ONE 25miler. Lots of people do trail in dressage saddles; my own preference for trail is a jumping-type saddle because it allows you to change position easily- stand up in 2-point, sit, post; changing position a lot keeps the fatigue from setting in. The synthetic saddles are great for trail- if a wintec pro jump with a wide gullet will fit your arab (fits mine like a glove) that's what I would suggest trying.