My morgan has almost no withers and the saddle doesn't slip on him. Treeless saddles CAN tend to slip, but if you use the right pads, that helps a great deal. I personally have a sheepskin bottomed pad made by a company called Christ (yeah, I know!), but there are others. I haven't tried using a "sticky" pad under the regular pad, but some people do. I'm sure there is a Freeform distributor in the UK -- you could see about doing a trial...they offer that here.
I really wouldn't recommend serious jumping in a treeless, however, as I mentioned in my blog post. When I was still jumping Gryphon (he was very round at the time...a little overweight!), the best saddle I found for him was a County. Fit him great, though for me, the balance wasn't as good as my Pessoa. That's very individual, though, so it might work great for you. Very nicely made saddles and they come really wide.
Keep me posted on how you do in your search!