How did I miss this little gem of a thread?
Respectfully, I'll have to disagree with you on barrel length being a requirement to achieve high velocities with a .20 cal cartridge.
I've been shooting a 20 Tactical since....let's see...1999/2000 (or somewhere thereabouts). Long before the 204R was even a thought. Back then Douglas was the only maker of .20 caliber blanks, so it was a rare thing indeed.
In a 20" barrel, with 33gr VMax (yes, they used to only offer 33gr; then 32gr and 40gr later on), I was seeing north of 4200fps (H-322 powder). Now granted, the .223 based case for the .20 Tactical is more efficient (especially with the thick Lake City brass), but high velocities can be achieved with proper powder selection (not necessarily longer barrel length).
The biggest hold up with the .20 cals is bullet selection. That's it. (But the offerings have grown dramatically since the early 2000's)
As far as terminal effects, it tends to outperform a standard .223 within normal hunting ranges due to the frangibility of the bullets and high velocity (or high RPM's to be exact, which is what causes the bullet to fly apart once upset/deformed). When I say "outperform" I am using that term in the context of small, fur bearing canids, where often times the pelt is sought after for bounty rewards, or for sale to the furrier in the spring. Extra holes (exits), require extra thread (and time). Hence the whole sub caliber craze in the late 90's/early 2000's; fur prices were up, and if you lived in the right area of the US, you could make a tidy sum in the winter (enough to pay for a new rifle at the end of the season). A prime wolf pelt would fetch $600, green and cased. A completely black wolf pelt would easily fetch $1000. And so we saw a dirge of subcaliber wildcats pop up. But I digress...
The .204R was designed for fox/coyote/wolves, and, to be very easy on the pelts. That may not be applicable in the UK (the pelt issue), but the fact it was intended for canids kind of answers the basic question; Is this good for fox/vermin? Yes.
It is no small irony that the 204R was originally based on a .223 case when on the drawing board at Hornady (IIRC, Dave Emery worked on this as one of his first projects there). But, as they say, a series of mistakes were made (assumptions really), and it ended up being based on the .222M case instead. Todd Kindler (creator of the .20 Tactical, 20 Vartarg and 20 TNT) used to lament about it. But that is another story entirely.
Bottom line: The .204R is a great cartridge, but has very specific advantages over the .223 (wind drift and violent expansion). That's not to say a .223 can't get the same job done, just that the 204R tends to excel at it, rather than just get the job done like a .223. Some people like scalpels, some people like cleavers. <shrug> It's the shooter's choice.