The problem we have is no matter what pellet you use there is simply not enough energy at 12ftlbs to cause the massive trauma required to dispatch a quarry, so your reliant on placing a shot in the kill zone which will cause enough damage to the brain to kill instantly, which on a rabbit is roughly a 25mm zone behind the eye, made worse by small pellets that over penetrate, there was a sound reasoning behind the .22 for fur .177 for feather rule of thumb.

If your fieldcraft is good you can get close benefitting in both velocity and accuracy, and to me this is the real issue, just because your rifle shoots good groups at 50yds doesn't mean you should be shooting live quarry at that range, when you shot those groups you weren't balancing uncomfortably on the side of a bank or between branches, a light puff of a breeze can be enough to put your shot out of the kill zone at that range and you have no control over the wind, and pretending a heavy pellet is wind proof is nonsense as recent testing I have been doing is proving.

If you keep your ranges down to 40yds and under if you cant consistently hit a 25mm target at that range, use a fast moving lighter pellet that gives up it's as much of it's energy on impact and only take shots you are sure of then 12ftlbs is and has been adequate for quarry in the UK, tens of thousands of vermin and game are testament to that fact.

But we really should consider 12ftlbs rifles to be a minimum requirement because your pellet may be carrying as little as 6ftlbs when it gets to the target, and no one is going to pretend hitting a rabbit with 20ftlbs is not going to be preferable.

Keep your ranges short and shoot from positions you are stable in, or your going to leave an animal to die suffering.