You don’t say if it’s wet felting or needlefelting you are wanting to do. Some breeds are good for one but not the other.
Blue-faced Leicester (BFL) doesn’t needlefelt very well, I believe, but wet felts beautifully. In my experience, it is the absolute prince of wet felting fleeces. I did a workshop where I used BFL and everyone else used merino. My item was solid in a very few minutes while everyone else worked up sweats, getting hot and bothered, for half an hour or more. I almost felt like I was cheating!

Ryeland needlefelts really well but is fairly resistant to wet felting.
Shetland wet felts well (although not as nearly effortlessly as BFL, in my experience) and is shorter staple than BFL, as you say you want a shorter staple.
Breeds resistant to wet felting include Texel, Cheviot, many of the Downs breeds.
I think all the Longwools will wet-felt fairly easily - Wensleydale, Teeswater, Leicester Longwool, Lincoln Longwool as well as BFL certainly do. (You can of course cut the locks in half or even thirds if you want shorter staples.)
Manx is a short staple and will both wet felt and needlefelt.
I’ve never tried to wet-felt Castlemilk Moorit and I’ve never had it felt on me (or on the sheep) - and I have handled it fairly roughly whilst washing and dyeing.
I haven’t specifically tried wet-felting Portland. If I had to have a punt, based on its handling during washing and dyeing, I’d suspect it’s not the best for wet felting, but I haven’t specifically tried.
Zwartbles fleeces are indeed highly variable; I’ve had fleeces from different Zwartbles sheep that were so different to each other (in every respect except colour) that you’d have thought they were from different breeds. Some would have wet-felted well, I think, and others were totally felting-resistant. (As a milk and meat sheep I guess there’s no reason it should have a fleece standard.)
Your Gotland will certainly wet-felt, although of course like the Zwartbles it isn’t a British breed.
Hebridean fleeces are famous for felting on the sheep’s backs, so if you can get them off the sheep unfelted I imagine you could then wet-felt. Some Hebs are double-fleeced, the shorter underlayer being finer and softer and much more prone to felting than the coarser, longer outer layer. You can separate the two layers fairly easily as you prepare the locks for carding.
Despite what one would expect, some of the northern blackface fleeces will felt. The coarsest of them all is the Rough Fell, and yet I managed to felt a Rough Fell fleece as I assumed that, being a coarse hill sheep’s fleece, it would not be prone to felting, and I handled it too roughly while washing it in the bath. But most of these types have the double coat, like the Hebridean, to a greater or lesser degree, and the softer, shorter, finer underlayer usually wet felts extremely easily.
I’ve a friend who uses fleece from her own flock to make felt. She makes shoes, purses, hats, brooches, and all sorts. She has BFLs, Shetlands and crossbreeds. Her absolute favourites for the fleeces for felting have some BFL and some Shetland in them, and she has a couple with a bit of Teeswater in there too.
Sorry, I’m a bit passionate about British sheep and their fleeces...
