Yeah, I ran across British ones as well (and also noted that they looked stout). It's hard to compare units without listing all the specs (weight provides a bit of a clue). Sadly, if you can't a get a hold of a certain brand/model then it's like it doesn't exist: and should you have to deal with getting parts?
EVERY unit has limitations. Knowing what they are and operating within those is the trick. My flail is a whole lot more delicate than my rotary cutter; I have to make myself operate the flail with that understanding (if it was a less heavy duty model I'd have to adjust for that as well).
I was pretty sure I wanted/needed a flail (desired extra cutting width [less passes means less turns means quicker] and the ability the shift the flail for cutting along ditches and in to light brush). Never used a flail before. The top level flails that met my basic requirements were around $6k. The thought that I could shell out $6k (or more) and find that I didn't like the flail made me apprehensive. I usually figure I can turn around and sell something, but in my area flails, for CUTs, aren't very common; there may not be a market (when you're talking about something that weighs 1,200 lbs shipping tends to force you into a local market). The flail that I ended up with has specs that rival those of the expensive units, and almost 1/2 the price. I figure the worst that could happen by buying the less expensive flail is that I might find myself losing $1k to $2k. Could I absorb that kind of risk? With a more expensive unit I might end up losing twice that (it's relative). I don't expect this flail to last 10 years, but if it does (which it probably will) then great!
Lots of knowledge on flails in this thread:
Flail Mower - Let's talk flail mowers
Might be worth asking in that thread if folks have any familiarity with pull-behind units such as you're interested in. I don't know a lot about gasoline engines, but I do know that Honda and Kawasaki engines tend to be in commercial-grade equipment: I have a riding lawnmower with a Kawasaki engine; rental equipment I get usually has Honda engines.