Hi there.
There are many western saddles made especially for the Arab physique, Google Arab western saddle, or Arab tree etc and you should come up with alot of choice. I know Smith Bros sell them, as do Schneiders, and State Line tack, all of whom I believe ship to the UK.
Failing that, speak to westernsaddler on here, I am sure she can get you set up with the right saddle for your Arab horse.
Basically the Arab tree has more rock (curve from front to back) and more flare (at the front the tree curves out a little more to allow shoulder room) then your traditional QH tree. Having said that I know of a lot of Arabs that actually are very flat through the back and fit a QH tree well... depends on your horse as always! Just look for round skirts mainly, like on barrel saddles etc... that will keep the length down more so then the square skirts.
And do try to get it fitted by somebody who REALLY knows their stuff, (like westernsaddler), western saddles need as careful fitting as any, and you wouldn't want to ruin all the good work it sounds like you've been doing!
good luck and have fun!