With a spring or piston based rifle, you only need about 13" barrel length to obtain max velocity. The rest may actually contribute drag to slow it down.
The longer barrels are great for iron sights, because they create the longer radius between front and rear. If you're using optics, no advantage there.
Longer barrel makes the break barrel guns easier to cock, contributing leverage. Finally there is the issue of balance and feel, which can affect accuracy of the shooter... some like the more muzzle heavy feel. If you carry all day in woods or field, the lower weight of the carbine is nice.
I think Weihrauch is pretty much top drawer, along with FWB. The RWS-Diana guns are very nice. I think most can be de-tuned to FAC limits for the UK.
The CZ is a very nice suggestion, overlooked and hard to get in the States. Another excellent value in a similar vein is Cometa, from Spain. I know theirs are available with 12 fpe springs for the UK. They have nothing in common with the Gamo line, and have been a quality bargain for years (formerly private labeled by RWS). Excellent barrels and triggers, like the CZs.
Finally, while we can feel for shooting UK brothers limited to 12fpe for general use, we can only envy their attitude on silencers. They are free and easy and generally available, where in the USA they are on the dark side of the gray area (unless built into the barrel assembly and non-removable from airguns).
