I read through the thread titled multiple questions and noted these posts, partially quoted below:
As you may well be aware, the UK has a limit on power for air rifles of 12 ft.lbs, above which a firearms certificate (FAC) is required for their ownership, this has been the case since 1968.
So under 12 ft.lbs of muzzle energy air rifles of any calibre are freely available to purchase. In essence this means 4.5mm (.177), 5mm (.20), 5.5mm (.22) and 6.35mm (.25) calibres.
The 5mm and 6.35mm have quite a limited following with few guns available and a fairly limited selection of pellets to choose from. In essence 4.5mm and 5.5mm are the main choices.
So given our power limitation which calibre has historically been the favourite in the UK?
Well, it is the 5.5mm, sure the 4.5mm has caught up in recent years and is the mainstay of the target disciplines (FT, HFT etc), but for the field shooter going after rabbits and the like the 5.5mm is still a strong favourite.
My first quality air rifle was an HW77K bought brand new in 1986, just like this one:
It was in 5.5mm it would run 14.5gr RWS Superdomes at a whisker under 600fps (JSB were not in existence at this time) and was nice and accurate at all sensible airgun hunting distances.
That 77 and other 5.5mm air rifles I have owned over the years harvested a lot of rabbits, so I would scarcely call them useless and no doubt this would echo the experiences of thousands of other airgun shooters in the UK over many years.
Taking the points Gerfrik makes, the 5.5mm is slower than a 4.5mm, so yes you do need to learn the trajectory and be good at assessing range to ensure clean kills, but that is just a matter of practice and any airgun is cheap enough to shoot to develop the appropriate skill level.
As to a "longer cycle time making hold sensitivity worse" - well that is nonsense, the difference in time between tripping the trigger and the pellet leaving the barrel is so small as to be a non-issue.
The 5.5mm air rifle is still a strong seller in the UK in both spring rifle and pre-charged pneumatic format, this does not support the idea of its being useless!
In terms of accuracy, use the right pellet and the 5.5mm is as accurate as you could want, even at the UK power limit - I once owned a BSA Spitfire in 5.5mm - this rifle is a strange sort of pcp that outwardly looks like a break barrel spring rifle:
This isn't my rifle but is an example of the type.
Despite being a mere 12ft.lbs rifle, it was stunningly accurate, grouping sub 25mm at 65 yards.
Even the mere springer, which is the primary subject of this thread can give stunning accuracy in 5.5mm and within the UK legal limit.
In pest control applications there are cases where the sub 12ft.lbs 5.5mm rifle is preferred - particularly at shorter distances for rats or when shooting inside buildings as the pellets tend to be less penetrating giving cleaner kills and less likelihood of damage.
So to sum up, the sub 12ft.lbs 5.5mm air rifle whether springer or pcp is not as useless as some would make it out to be, it does take more effort to learn though and this might be offputting to those who don't have the patience to learn.